diff --git a/news/125.dep b/news/125.dep
new file mode 100644
index 0000000000000000000000000000000000000000..1ad090098b828e61324efabfe518dd8f7cd3aab9
--- /dev/null
+++ b/news/125.dep
@@ -0,0 +1 @@
+Upgrade dependency `pip` to `20.1`.
diff --git a/pdm.lock b/pdm.lock
index cc8dcde1b2fb6f96797184884dee142114b2c46d..0f0aa224e368156845551dfb81a8a80695933920 100644
--- a/pdm.lock
+++ b/pdm.lock
@@ -8,7 +8,7 @@ summary = "apipkg: namespace control and lazy-import mechanism"
 [[package]]
 name = "appdirs"
 sections = ["default"]
-version = "1.4.3"
+version = "1.4.4"
 summary = "A small Python module for determining appropriate platform-specific dirs, e.g. a \"user data dir\"."
 
 [[package]]
@@ -98,8 +98,8 @@ six = ">=1.12.0"
 
 [[package]]
 name = "importlib-metadata"
-sections = ["dev"]
-version = "1.6.0"
+sections = ["dev", "doc"]
+version = "1.6.1"
 marker = "python_version >= '3.5' and python_version < '3.8'"
 summary = "Read metadata from Python packages"
 
@@ -110,7 +110,7 @@ zipp = ">=0.5"
 name = "incremental"
 sections = ["dev"]
 version = "17.5.0"
-summary = "UNKNOWN"
+summary = ""
 
 [[package]]
 name = "jinja2"
@@ -125,20 +125,20 @@ MarkupSafe = ">=0.23"
 [[package]]
 name = "joblib"
 sections = ["doc"]
-version = "0.14.1"
-marker = "python_version >= '3.5'"
+version = "0.15.1"
+marker = "python_version >= '3.6'"
 summary = "Lightweight pipelining: using Python functions as pipeline jobs."
 
 [[package]]
 name = "livereload"
 sections = ["doc"]
-version = "2.6.1"
+version = "2.6.2"
 marker = "python_version >= '3.5'"
 summary = "Python LiveReload is an awesome tool for web developers"
 
 [package.dependencies]
-tornado = "*"
 six = "*"
+tornado = {marker = "python_version > '2.7'", version = "*"}
 
 [[package]]
 name = "log-symbols"
@@ -152,7 +152,7 @@ colorama = ">=0.3.9"
 [[package]]
 name = "lunr"
 sections = ["doc"]
-version = "0.5.6"
+version = "0.5.8"
 marker = "python_version >= '3.5'"
 summary = "A Python implementation of Lunr.js"
 
@@ -163,7 +163,7 @@ six = ">=1.11.0"
 [[package]]
 name = "lunr"
 sections = ["doc"]
-version = "0.5.6"
+version = "0.5.8"
 extras = ["languages"]
 marker = "python_version >= '3.5'"
 summary = "A Python implementation of Lunr.js"
@@ -171,18 +171,18 @@ summary = "A Python implementation of Lunr.js"
 [package.dependencies]
 future = ">=0.16.0"
 six = ">=1.11.0"
-nltk = ">=3.2.5"
-lunr = {marker = "python_version >= '3.5'", version = "==0.5.6"}
+nltk = {marker = "python_version > '2.7'", version = ">=3.2.5"}
+lunr = "==0.5.8"
 
 [[package]]
 name = "markdown"
 sections = ["doc"]
-version = "3.2.1"
+version = "3.2.2"
 marker = "python_version >= '3.5'"
 summary = "Python implementation of Markdown."
 
 [package.dependencies]
-setuptools = ">=36"
+importlib-metadata = {marker = "python_version < '3.8'", version = "*"}
 
 [[package]]
 name = "markdown-include"
@@ -190,9 +190,6 @@ sections = ["doc"]
 version = "0.5.1"
 summary = "This is an extension to Python-Markdown which provides an \"include\" function, similar to that found in LaTeX (and also the C pre-processor and Fortran). I originally wrote it for my FORD Fortran auto-documentation generator."
 
-[package.dependencies]
-markdown = "*"
-
 [[package]]
 name = "MarkupSafe"
 sections = ["dev", "doc"]
@@ -203,7 +200,7 @@ summary = "Safely add untrusted strings to HTML/XML markup."
 [[package]]
 name = "mkdocs"
 sections = ["doc"]
-version = "1.1"
+version = "1.1.2"
 marker = "python_version >= '3.5'"
 summary = "Project documentation with Markdown."
 
@@ -211,7 +208,7 @@ summary = "Project documentation with Markdown."
 click = ">=3.3"
 Jinja2 = ">=2.10.1"
 livereload = ">=2.5.1"
-lunr = {extras = ["languages"], version = "==0.5.6"}
+lunr = {extras = ["languages"], version = "==0.5.8"}
 Markdown = ">=3.2.1"
 PyYAML = ">=3.10"
 tornado = ">=5.0"
@@ -231,7 +228,7 @@ pymdown-extensions = ">=6.3"
 [[package]]
 name = "more-itertools"
 sections = ["dev"]
-version = "8.2.0"
+version = "8.3.0"
 marker = "python_version >= '3.5'"
 summary = "More routines for operating on iterables, beyond itertools"
 
@@ -251,7 +248,7 @@ tqdm = "*"
 [[package]]
 name = "packaging"
 sections = ["default", "dev"]
-version = "20.3"
+version = "20.4"
 marker = "python_version >= '2.7'"
 summary = "Core utilities for Python packages"
 
@@ -262,7 +259,7 @@ six = "*"
 [[package]]
 name = "pip"
 sections = ["default"]
-version = "20.0.2"
+version = "20.1.1"
 marker = "python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3, 3.4'"
 summary = "The PyPA recommended tool for installing Python packages."
 
@@ -324,7 +321,7 @@ summary = "Python parsing module"
 [[package]]
 name = "pytest"
 sections = ["dev"]
-version = "5.4.1"
+version = "5.4.3"
 marker = "python_version >= '3.5'"
 summary = "pytest: simple powerful testing with Python"
 
@@ -342,8 +339,8 @@ colorama = {marker = "sys_platform == 'win32'", version = "*"}
 [[package]]
 name = "pytest-cov"
 sections = ["dev"]
-version = "2.8.1"
-marker = "python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'"
+version = "2.9.0"
+marker = "python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3, 3.4'"
 summary = "Pytest plugin for measuring coverage."
 
 [package.dependencies]
@@ -363,7 +360,7 @@ pytest = ">=3.1.0"
 [[package]]
 name = "pytest-mock"
 sections = ["dev"]
-version = "3.1.0"
+version = "3.1.1"
 marker = "python_version >= '3.5'"
 summary = "Thin-wrapper around the mock package for easier use with pytest"
 
@@ -396,7 +393,7 @@ colorama = "*"
 [[package]]
 name = "pythonfinder"
 sections = ["default"]
-version = "1.2.3"
+version = "1.2.4"
 marker = "python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'"
 summary = "A cross-platform python discovery tool to help locate python on any system."
 
@@ -417,7 +414,7 @@ summary = "YAML parser and emitter for Python"
 [[package]]
 name = "regex"
 sections = ["doc"]
-version = "2020.4.4"
+version = "2020.6.8"
 marker = "python_version >= '3.5'"
 summary = "Alternative regular expression module, to replace re."
 
@@ -429,15 +426,15 @@ summary = "Resolve abstract dependencies into concrete ones"
 
 [[package]]
 name = "setuptools"
-sections = ["default", "doc"]
-version = "46.1.3"
+sections = ["default"]
+version = "47.1.1"
 marker = "python_version >= '3.5'"
 summary = "Easily download, build, install, upgrade, and uninstall Python packages"
 
 [[package]]
 name = "six"
 sections = ["default", "dev", "doc"]
-version = "1.14.0"
+version = "1.15.0"
 marker = "python_version >= '2.7'"
 summary = "Python 2 and 3 compatibility utilities"
 
@@ -456,7 +453,7 @@ summary = "ANSII Color formatting for output in terminal."
 [[package]]
 name = "toml"
 sections = ["dev"]
-version = "0.10.0"
+version = "0.10.1"
 summary = "Python Library for Tom's Obvious, Minimal Language"
 
 [[package]]
@@ -488,16 +485,16 @@ toml = "*"
 [[package]]
 name = "tqdm"
 sections = ["doc"]
-version = "4.46.0"
+version = "4.46.1"
 marker = "python_version >= '3.5'"
 summary = "Fast, Extensible Progress Meter"
 
 [[package]]
 name = "wcwidth"
 sections = ["dev"]
-version = "0.1.9"
+version = "0.2.4"
 marker = "python_version >= '3.5'"
-summary = "Measures number of Terminal column cells of wide-character codes"
+summary = "Measures the displayed width of unicode strings in a terminal"
 
 [[package]]
 name = "wheel"
@@ -508,7 +505,7 @@ summary = "A built-package format for Python"
 
 [[package]]
 name = "zipp"
-sections = ["dev"]
+sections = ["dev", "doc"]
 version = "3.1.0"
 marker = "python_version >= '3.6' and python_version < '3.8'"
 summary = "Backport of pathlib-compatible object wrapper for zip files"
@@ -518,9 +515,9 @@ summary = "Backport of pathlib-compatible object wrapper for zip files"
     {file = "apipkg-1.5-py2.py3-none-any.whl", hash = "sha256:58587dd4dc3daefad0487f6d9ae32b4542b185e1c36db6993290e7c41ca2b47c"},
     {file = "apipkg-1.5.tar.gz", hash = "sha256:37228cda29411948b422fae072f57e31d3396d2ee1c9783775980ee9c9990af6"},
 ]
-"appdirs 1.4.3" = [
-    {file = "appdirs-1.4.3-py2.py3-none-any.whl", hash = "sha256:d8b24664561d0d34ddfaec54636d502d7cea6e29c3eaf68f3df6180863e2166e"},
-    {file = "appdirs-1.4.3.tar.gz", hash = "sha256:9e5896d1372858f8dd3344faf4e5014d21849c756c8d5701f78f8a103b372d92"},
+"appdirs 1.4.4" = [
+    {file = "appdirs-1.4.4-py2.py3-none-any.whl", hash = "sha256:a841dacd6b99318a741b166adb07e19ee71a274450e68237b4650ca1055ab128"},
+    {file = "appdirs-1.4.4.tar.gz", hash = "sha256:7d5d0167b2b1ba821647616af46a749d1c653740dd0d2415100fe26e27afdf41"},
 ]
 "atomicwrites 1.4.0" = [
     {file = "atomicwrites-1.4.0-py2.py3-none-any.whl", hash = "sha256:6d1784dea7c0c8d4a5172b6c620f40b6e4cbfdf96d783691f2e1302a7b88e197"},
@@ -592,9 +589,9 @@ summary = "Backport of pathlib-compatible object wrapper for zip files"
     {file = "halo-0.0.29-py3-none-any.whl", hash = "sha256:67996649083229a9b7a93fe7c871c97d40bd2f1a988540eccf948a6b1844f644"},
     {file = "halo-0.0.29.tar.gz", hash = "sha256:a8aeb0164e269d7c96fb7444b2a4caaa09b8989fa0c85e6a26e8b2c6d1af3b9d"},
 ]
-"importlib-metadata 1.6.0" = [
-    {file = "importlib_metadata-1.6.0-py2.py3-none-any.whl", hash = "sha256:2a688cbaa90e0cc587f1df48bdc97a6eadccdcd9c35fb3f976a09e3b5016d90f"},
-    {file = "importlib_metadata-1.6.0.tar.gz", hash = "sha256:34513a8a0c4962bc66d35b359558fd8a5e10cd472d37aec5f66858addef32c1e"},
+"importlib-metadata 1.6.1" = [
+    {file = "importlib_metadata-1.6.1-py2.py3-none-any.whl", hash = "sha256:15ec6c0fd909e893e3a08b3a7c76ecb149122fb14b7efe1199ddd4c7c57ea958"},
+    {file = "importlib_metadata-1.6.1.tar.gz", hash = "sha256:0505dd08068cfec00f53a74a0ad927676d7757da81b7436a6eefe4c7cf75c545"},
 ]
 "incremental 17.5.0" = [
     {file = "incremental-17.5.0-py2.py3-none-any.whl", hash = "sha256:717e12246dddf231a349175f48d74d93e2897244939173b01974ab6661406b9f"},
@@ -604,29 +601,28 @@ summary = "Backport of pathlib-compatible object wrapper for zip files"
     {file = "Jinja2-2.11.2-py2.py3-none-any.whl", hash = "sha256:f0a4641d3cf955324a89c04f3d94663aa4d638abe8f733ecd3582848e1c37035"},
     {file = "Jinja2-2.11.2.tar.gz", hash = "sha256:89aab215427ef59c34ad58735269eb58b1a5808103067f7bb9d5836c651b3bb0"},
 ]
-"joblib 0.14.1" = [
-    {file = "joblib-0.14.1-py2.py3-none-any.whl", hash = "sha256:bdb4fd9b72915ffb49fde2229ce482dd7ae79d842ed8c2b4c932441495af1403"},
-    {file = "joblib-0.14.1.tar.gz", hash = "sha256:0630eea4f5664c463f23fbf5dcfc54a2bc6168902719fa8e19daf033022786c8"},
+"joblib 0.15.1" = [
+    {file = "joblib-0.15.1-py3-none-any.whl", hash = "sha256:6825784ffda353cc8a1be573118085789e5b5d29401856b35b756645ab5aecb5"},
+    {file = "joblib-0.15.1.tar.gz", hash = "sha256:61e49189c84b3c5d99a969d314853f4d1d263316cc694bec17548ebaa9c47b6e"},
 ]
-"livereload 2.6.1" = [
-    {file = "livereload-2.6.1-py2.py3-none-any.whl", hash = "sha256:78d55f2c268a8823ba499305dcac64e28ddeb9a92571e12d543cd304faf5817b"},
-    {file = "livereload-2.6.1.tar.gz", hash = "sha256:89254f78d7529d7ea0a3417d224c34287ebfe266b05e67e51facaf82c27f0f66"},
+"livereload 2.6.2" = [
+    {file = "livereload-2.6.2.tar.gz", hash = "sha256:d1eddcb5c5eb8d2ca1fa1f750e580da624c0f7fcb734aa5780dc81b7dcbd89be"},
 ]
 "log-symbols 0.0.14" = [
     {file = "log_symbols-0.0.14-py3-none-any.whl", hash = "sha256:4952106ff8b605ab7d5081dd2c7e6ca7374584eff7086f499c06edd1ce56dcca"},
     {file = "log_symbols-0.0.14.tar.gz", hash = "sha256:cf0bbc6fe1a8e53f0d174a716bc625c4f87043cc21eb55dd8a740cfe22680556"},
 ]
-"lunr 0.5.6" = [
-    {file = "lunr-0.5.6-py2.py3-none-any.whl", hash = "sha256:1208622930c915a07e6f8e8640474357826bad48534c0f57969b6fca9bffc88e"},
-    {file = "lunr-0.5.6.tar.gz", hash = "sha256:7be69d7186f65784a4f2adf81e5c58efd6a9921aa95966babcb1f2f2ada75c20"},
+"lunr 0.5.8" = [
+    {file = "lunr-0.5.8-py2.py3-none-any.whl", hash = "sha256:aab3f489c4d4fab4c1294a257a30fec397db56f0a50273218ccc3efdbf01d6ca"},
+    {file = "lunr-0.5.8.tar.gz", hash = "sha256:c4fb063b98eff775dd638b3df380008ae85e6cb1d1a24d1cd81a10ef6391c26e"},
 ]
-"lunr[languages] 0.5.6" = [
-    {file = "lunr-0.5.6-py2.py3-none-any.whl", hash = "sha256:1208622930c915a07e6f8e8640474357826bad48534c0f57969b6fca9bffc88e"},
-    {file = "lunr-0.5.6.tar.gz", hash = "sha256:7be69d7186f65784a4f2adf81e5c58efd6a9921aa95966babcb1f2f2ada75c20"},
+"lunr[languages] 0.5.8" = [
+    {file = "lunr-0.5.8-py2.py3-none-any.whl", hash = "sha256:aab3f489c4d4fab4c1294a257a30fec397db56f0a50273218ccc3efdbf01d6ca"},
+    {file = "lunr-0.5.8.tar.gz", hash = "sha256:c4fb063b98eff775dd638b3df380008ae85e6cb1d1a24d1cd81a10ef6391c26e"},
 ]
-"markdown 3.2.1" = [
-    {file = "Markdown-3.2.1-py2.py3-none-any.whl", hash = "sha256:e4795399163109457d4c5af2183fbe6b60326c17cfdf25ce6e7474c6624f725d"},
-    {file = "Markdown-3.2.1.tar.gz", hash = "sha256:90fee683eeabe1a92e149f7ba74e5ccdc81cd397bd6c516d93a8da0ef90b6902"},
+"markdown 3.2.2" = [
+    {file = "Markdown-3.2.2-py3-none-any.whl", hash = "sha256:c467cd6233885534bf0fe96e62e3cf46cfc1605112356c4f9981512b8174de59"},
+    {file = "Markdown-3.2.2.tar.gz", hash = "sha256:1fafe3f1ecabfb514a5285fca634a53c1b32a81cb0feb154264d55bf2ff22c17"},
 ]
 "markdown-include 0.5.1" = [
     {file = "markdown-include-0.5.1.tar.gz", hash = "sha256:72a45461b589489a088753893bc95c5fa5909936186485f4ed55caa57d10250f"},
@@ -666,28 +662,28 @@ summary = "Backport of pathlib-compatible object wrapper for zip files"
     {file = "MarkupSafe-1.1.1-cp38-cp38-win_amd64.whl", hash = "sha256:e8313f01ba26fbbe36c7be1966a7b7424942f670f38e666995b88d012765b9be"},
     {file = "MarkupSafe-1.1.1.tar.gz", hash = "sha256:29872e92839765e546828bb7754a68c418d927cd064fd4708fab9fe9c8bb116b"},
 ]
-"mkdocs 1.1" = [
-    {file = "mkdocs-1.1-py2.py3-none-any.whl", hash = "sha256:1e385a70aea8a9dedb731aea4fd5f3704b2074801c4f96f06b2920999babda8a"},
-    {file = "mkdocs-1.1.tar.gz", hash = "sha256:9243291392f59e20b655e4e46210233453faf97787c2cf72176510e868143174"},
+"mkdocs 1.1.2" = [
+    {file = "mkdocs-1.1.2-py3-none-any.whl", hash = "sha256:096f52ff52c02c7e90332d2e53da862fde5c062086e1b5356a6e392d5d60f5e9"},
+    {file = "mkdocs-1.1.2.tar.gz", hash = "sha256:f0b61e5402b99d7789efa032c7a74c90a20220a9c81749da06dbfbcbd52ffb39"},
 ]
 "mkdocs-material 4.6.3" = [
     {file = "mkdocs_material-4.6.3-py2.py3-none-any.whl", hash = "sha256:7f3afa0a09c07d0b89a6a9755fdb00513aee8f0cec3538bb903325c80f66f444"},
     {file = "mkdocs-material-4.6.3.tar.gz", hash = "sha256:1d486635b03f5a2ec87325842f7b10c7ae7daa0eef76b185572eece6a6ea212c"},
 ]
-"more-itertools 8.2.0" = [
-    {file = "more_itertools-8.2.0-py3-none-any.whl", hash = "sha256:5dd8bcf33e5f9513ffa06d5ad33d78f31e1931ac9a18f33d37e77a180d393a7c"},
-    {file = "more-itertools-8.2.0.tar.gz", hash = "sha256:b1ddb932186d8a6ac451e1d95844b382f55e12686d51ca0c68b6f61f2ab7a507"},
+"more-itertools 8.3.0" = [
+    {file = "more_itertools-8.3.0-py3-none-any.whl", hash = "sha256:7818f596b1e87be009031c7653d01acc46ed422e6656b394b0f765ce66ed4982"},
+    {file = "more-itertools-8.3.0.tar.gz", hash = "sha256:558bb897a2232f5e4f8e2399089e35aecb746e1f9191b6584a151647e89267be"},
 ]
 "nltk 3.5" = [
     {file = "nltk-3.5.zip", hash = "sha256:845365449cd8c5f9731f7cb9f8bd6fd0767553b9d53af9eb1b3abf7700936b35"},
 ]
-"packaging 20.3" = [
-    {file = "packaging-20.3-py2.py3-none-any.whl", hash = "sha256:82f77b9bee21c1bafbf35a84905d604d5d1223801d639cf3ed140bd651c08752"},
-    {file = "packaging-20.3.tar.gz", hash = "sha256:3c292b474fda1671ec57d46d739d072bfd495a4f51ad01a055121d81e952b7a3"},
+"packaging 20.4" = [
+    {file = "packaging-20.4-py2.py3-none-any.whl", hash = "sha256:998416ba6962ae7fbd6596850b80e17859a5753ba17c32284f67bfff33784181"},
+    {file = "packaging-20.4.tar.gz", hash = "sha256:4357f74f47b9c12db93624a82154e9b120fa8293699949152b22065d556079f8"},
 ]
-"pip 20.0.2" = [
-    {file = "pip-20.0.2-py2.py3-none-any.whl", hash = "sha256:4ae14a42d8adba3205ebeb38aa68cfc0b6c346e1ae2e699a0b3bad4da19cef5c"},
-    {file = "pip-20.0.2.tar.gz", hash = "sha256:7db0c8ea4c7ea51c8049640e8e6e7fde949de672bfa4949920675563a5a6967f"},
+"pip 20.1.1" = [
+    {file = "pip-20.1.1-py2.py3-none-any.whl", hash = "sha256:b27c4dedae8c41aa59108f2fa38bf78e0890e590545bc8ece7cdceb4ba60f6e4"},
+    {file = "pip-20.1.1.tar.gz", hash = "sha256:27f8dc29387dd83249e06e681ce087e6061826582198a425085e0bf4c1cf3a55"},
 ]
 "pip-shims 0.5.2" = [
     {file = "pip_shims-0.5.2-py2.py3-none-any.whl", hash = "sha256:39193b8c4aa5e4cb82e250be58df4d5eaebe931a33b0df43b369f4ae92ee5753"},
@@ -713,21 +709,21 @@ summary = "Backport of pathlib-compatible object wrapper for zip files"
     {file = "pyparsing-2.4.7-py2.py3-none-any.whl", hash = "sha256:ef9d7589ef3c200abe66653d3f1ab1033c3c419ae9b9bdb1240a85b024efc88b"},
     {file = "pyparsing-2.4.7.tar.gz", hash = "sha256:c203ec8783bf771a155b207279b9bccb8dea02d8f0c9e5f8ead507bc3246ecc1"},
 ]
-"pytest 5.4.1" = [
-    {file = "pytest-5.4.1-py3-none-any.whl", hash = "sha256:0e5b30f5cb04e887b91b1ee519fa3d89049595f428c1db76e73bd7f17b09b172"},
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+"pytest 5.4.3" = [
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 ]
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 ]
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@@ -737,9 +733,9 @@ summary = "Backport of pathlib-compatible object wrapper for zip files"
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 ]
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 ]
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@@ -754,40 +750,40 @@ summary = "Backport of pathlib-compatible object wrapper for zip files"
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 ]
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+"regex 2020.6.8" = [
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+    {file = "regex-2020.6.8.tar.gz", hash = "sha256:e9b64e609d37438f7d6e68c2546d2cb8062f3adb27e6336bc129b51be20773ac"},
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 "resolvelib 0.4.0" = [
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     {file = "resolvelib-0.4.0.tar.gz", hash = "sha256:93e42cf712534cf8dccb146c0c20521a5ac344ef8ec6e7742a49b22a24335662"},
 ]
-"setuptools 46.1.3" = [
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-    {file = "setuptools-46.1.3.zip", hash = "sha256:795e0475ba6cd7fa082b1ee6e90d552209995627a2a227a47c6ea93282f4bfb1"},
+"setuptools 47.1.1" = [
+    {file = "setuptools-47.1.1-py3-none-any.whl", hash = "sha256:74f33f44290f95c5c4a7c13ccc9d6d1a16837fe9dce0acf411dd244e7de95143"},
+    {file = "setuptools-47.1.1.zip", hash = "sha256:145fa62b9d7bb544fce16e9b5a9bf4ab2032d2f758b7cd674af09a92736aff74"},
 ]
-"six 1.14.0" = [
-    {file = "six-1.14.0-py2.py3-none-any.whl", hash = "sha256:8f3cd2e254d8f793e7f3d6d9df77b92252b52637291d0f0da013c76ea2724b6c"},
-    {file = "six-1.14.0.tar.gz", hash = "sha256:236bdbdce46e6e6a3d61a337c0f8b763ca1e8717c03b369e87a7ec7ce1319c0a"},
+"six 1.15.0" = [
+    {file = "six-1.15.0-py2.py3-none-any.whl", hash = "sha256:8b74bedcbbbaca38ff6d7491d76f2b06b3592611af620f8426e82dddb04a5ced"},
+    {file = "six-1.15.0.tar.gz", hash = "sha256:30639c035cdb23534cd4aa2dd52c3bf48f06e5f4a941509c8bafd8ce11080259"},
 ]
 "spinners 0.0.24" = [
     {file = "spinners-0.0.24-py3-none-any.whl", hash = "sha256:2fa30d0b72c9650ad12bbe031c9943b8d441e41b4f5602b0ec977a19f3290e98"},
@@ -796,9 +792,9 @@ summary = "Backport of pathlib-compatible object wrapper for zip files"
 "termcolor 1.1.0" = [
     {file = "termcolor-1.1.0.tar.gz", hash = "sha256:1d6d69ce66211143803fbc56652b41d73b4a400a2891d7bf7a1cdf4c02de613b"},
 ]
-"toml 0.10.0" = [
-    {file = "toml-0.10.0-py2.py3-none-any.whl", hash = "sha256:235682dd292d5899d361a811df37e04a8828a5b1da3115886b73cf81ebc9100e"},
-    {file = "toml-0.10.0.tar.gz", hash = "sha256:229f81c57791a41d65e399fc06bf0848bab550a9dfd5ed66df18ce5f05e73d5c"},
+"toml 0.10.1" = [
+    {file = "toml-0.10.1-py2.py3-none-any.whl", hash = "sha256:bda89d5935c2eac546d648028b9901107a595863cb36bae0c73ac804a9b4ce88"},
+    {file = "toml-0.10.1.tar.gz", hash = "sha256:926b612be1e5ce0634a2ca03470f95169cf16f939018233a670519cb4ac58b0f"},
 ]
 "tomlkit 0.6.0" = [
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@@ -819,13 +815,13 @@ summary = "Backport of pathlib-compatible object wrapper for zip files"
     {file = "towncrier-19.2.0-py2.py3-none-any.whl", hash = "sha256:de19da8b8cb44f18ea7ed3a3823087d2af8fcf497151bb9fd1e1b092ff56ed8d"},
     {file = "towncrier-19.2.0.tar.gz", hash = "sha256:48251a1ae66d2cf7e6fa5552016386831b3e12bb3b2d08eb70374508c17a8196"},
 ]
-"tqdm 4.46.0" = [
-    {file = "tqdm-4.46.0-py2.py3-none-any.whl", hash = "sha256:acdafb20f51637ca3954150d0405ff1a7edde0ff19e38fb99a80a66210d2a28f"},
-    {file = "tqdm-4.46.0.tar.gz", hash = "sha256:4733c4a10d0f2a4d098d801464bdaf5240c7dadd2a7fde4ee93b0a0efd9fb25e"},
+"tqdm 4.46.1" = [
+    {file = "tqdm-4.46.1-py2.py3-none-any.whl", hash = "sha256:07c06493f1403c1380b630ae3dcbe5ae62abcf369a93bbc052502279f189ab8c"},
+    {file = "tqdm-4.46.1.tar.gz", hash = "sha256:cd140979c2bebd2311dfb14781d8f19bd5a9debb92dcab9f6ef899c987fcf71f"},
 ]
-"wcwidth 0.1.9" = [
-    {file = "wcwidth-0.1.9-py2.py3-none-any.whl", hash = "sha256:cafe2186b3c009a04067022ce1dcd79cb38d8d65ee4f4791b8888d6599d1bbe1"},
-    {file = "wcwidth-0.1.9.tar.gz", hash = "sha256:ee73862862a156bf77ff92b09034fc4825dd3af9cf81bc5b360668d425f3c5f1"},
+"wcwidth 0.2.4" = [
+    {file = "wcwidth-0.2.4-py2.py3-none-any.whl", hash = "sha256:79375666b9954d4a1a10739315816324c3e73110af9d0e102d906fdb0aec009f"},
+    {file = "wcwidth-0.2.4.tar.gz", hash = "sha256:8c6b5b6ee1360b842645f336d9e5d68c55817c26d3050f46b235ef2bc650e48f"},
 ]
 "wheel 0.34.2" = [
     {file = "wheel-0.34.2-py2.py3-none-any.whl", hash = "sha256:df277cb51e61359aba502208d680f90c0493adec6f0e848af94948778aed386e"},
@@ -837,5 +833,5 @@ summary = "Backport of pathlib-compatible object wrapper for zip files"
 ]
 
 [root]
-content_hash = "md5:327f6760ec53344e472d579a529ce7a8"
+content_hash = "md5:053be4630c93f8942df8ae712cf52681"
 meta_version = "0.0.1"
diff --git a/pdm/core.py b/pdm/core.py
index 548687b167eedee91d839eb22cce21d5c0373602..8c680d8f61c9583e5554f7e89471660ec21d5fd0 100644
--- a/pdm/core.py
+++ b/pdm/core.py
@@ -69,6 +69,8 @@ class Core:
 
     def main(self, args=None, prog_name=None, obj=None, **extra):
         """The main entry function"""
+        from pip._internal.utils.temp_dir import global_tempdir_manager
+
         self.init_parser()
         self.load_plugins()
 
@@ -93,7 +95,8 @@ class Core:
             sys.exit(1)
         else:
             try:
-                f(options.project, options)
+                with global_tempdir_manager():
+                    f(options.project, options)
             except Exception:
                 etype, err, traceback = sys.exc_info()
                 if stream.verbosity > stream.NORMAL:
diff --git a/pdm/formats/requirements.py b/pdm/formats/requirements.py
index 80ed3e235b177e1742a1fea2b591aec593dcbc29..37c230c8eab420dab9e877204074086b6fe605a6 100644
--- a/pdm/formats/requirements.py
+++ b/pdm/formats/requirements.py
@@ -55,8 +55,13 @@ def requirement_from_ireq(ireq):
 
 
 def parse_requirement_file(filename):
+    from pip._internal.req.constructors import install_req_from_parsed_requirement
+
     finder = get_finder([])
-    ireqs = list(parse_requirements(filename, finder.session, finder))
+    ireqs = [
+        install_req_from_parsed_requirement(pr)
+        for pr in parse_requirements(filename, finder.session, finder)
+    ]
     return ireqs, finder
 
 
diff --git a/pdm/models/environment.py b/pdm/models/environment.py
index 8a8cfc5c87683022e4ec74bb014d56274851f98d..b50ce9ee954b5a659c80b5bc8593bcd0cb5ce2d7 100644
--- a/pdm/models/environment.py
+++ b/pdm/models/environment.py
@@ -31,6 +31,7 @@ from pdm.utils import (
     get_python_version,
     get_sys_config_paths,
     get_venv_python,
+    populate_link,
     temp_environ,
 )
 
@@ -272,7 +273,6 @@ class Environment:
         :param allow_all: Allow building incompatible wheels.
         :returns: The full path of the built artifact.
         """
-        from pip._internal.utils.temp_dir import global_tempdir_manager
         from pdm.builders import EditableBuilder
         from pdm.builders import WheelBuilder
 
@@ -281,9 +281,9 @@ class Environment:
             if allow_all:
                 with allow_all_wheels():
                     # temporarily allow all wheels to get a link.
-                    ireq.populate_link(finder, False, bool(hashes))
+                    populate_link(finder, ireq, False)
             else:
-                ireq.populate_link(finder, False, bool(hashes))
+                populate_link(finder, ireq, False)
             if not ireq.editable and not ireq.req.name:
                 ireq.source_dir = kwargs["build_dir"]
             else:
@@ -295,23 +295,22 @@ class Environment:
                 download_dir = kwargs["wheel_download_dir"]
                 only_download = True
             if hashes:
-                ireq.options["hashes"] = convert_hashes(hashes)
+                ireq.hash_options = convert_hashes(hashes)
             if not (ireq.editable and ireq.req.is_local_dir):
-                with global_tempdir_manager():
-                    downloader = shims.Downloader(finder.session, "off")
-                    downloaded = shims.unpack_url(
-                        ireq.link,
-                        ireq.source_dir,
-                        downloader,
-                        download_dir,
-                        ireq.hashes(False),
-                    )
-                    # Preserve the downloaded file so that it won't be cleared.
-                    if downloaded and only_download:
-                        try:
-                            shutil.copy(downloaded, download_dir)
-                        except shutil.SameFileError:
-                            pass
+                downloader = shims.Downloader(finder.session, "off")
+                downloaded = shims.unpack_url(
+                    ireq.link,
+                    ireq.source_dir,
+                    downloader,
+                    download_dir,
+                    ireq.hashes(False),
+                )
+                # Preserve the downloaded file so that it won't be cleared.
+                if downloaded and only_download:
+                    try:
+                        shutil.copy(downloaded.path, download_dir)
+                    except shutil.SameFileError:
+                        pass
             # Now all source is prepared, build it.
             if ireq.link.is_wheel:
                 return (self.project.cache("wheels") / ireq.link.filename).as_posix()
diff --git a/pdm/utils.py b/pdm/utils.py
index ce089439258e4ed8977971615acd2680f5591030..b532f477bc08e16fe544fa3806c56bfa8681cb58 100644
--- a/pdm/utils.py
+++ b/pdm/utils.py
@@ -19,7 +19,13 @@ from typing import TYPE_CHECKING, Any, Dict, List, Optional, Tuple, Union
 
 from distlib.wheel import Wheel
 from packaging.version import parse as parse_version
-from pip_shims.shims import InstallCommand, PackageFinder, TargetPython, url_to_path
+from pip_shims.shims import (
+    InstallCommand,
+    InstallRequirement,
+    PackageFinder,
+    TargetPython,
+    url_to_path,
+)
 
 from pdm._types import Source
 
@@ -506,3 +512,13 @@ def get_platform():
 def highest_version(versions: List[str]) -> str:
     """Return the highest version of a given list."""
     return max(versions, key=parse_version)
+
+
+def populate_link(
+    finder: PackageFinder, ireq: InstallRequirement, upgrade: bool = False
+):
+    """Populate ireq's link attribute"""
+    if ireq.link:
+        return
+    link = finder.find_requirement(ireq, upgrade)
+    ireq.link = link
diff --git a/pyproject.toml b/pyproject.toml
index 4a2197864fc0b9e41088e5f3179b4874c6e698db..b11d13325b270c7f75f2658512aff06027a9b98e 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -27,7 +27,7 @@ Documentation = "https://pdm.fming.dev"
 appdirs = "*"
 click = "*"
 distlib = "*"
-pip = "<20.1,>=20.0"
+pip = ">=20.1"
 pip_shims = "*"
 pythonfinder = "*"
 tomlkit = "*"
diff --git a/setup_dev.py b/setup_dev.py
index 27ae841574433e9869a81514677af47409b838b6..537963516145a245a6d5f2ad299934a9efefd9bc 100644
--- a/setup_dev.py
+++ b/setup_dev.py
@@ -26,7 +26,7 @@ def main():
 
     print("Installing base requirements...", flush=True)
     subprocess.check_call(
-        [venv_python.as_posix(), "-m", "pip", "install", "-U", "pip<20.1", "pdm"]
+        [venv_python.as_posix(), "-m", "pip", "install", "-U", "pip", "pdm"]
     )
 
     print("Setup project for development...", flush=True)
diff --git a/tests/cli/test_actions.py b/tests/cli/test_actions.py
index b1dc8395fdebcef071b1605efe4a29354da9a6d2..685138ee398b8c6cdce75fb7302c2d454122f25b 100644
--- a/tests/cli/test_actions.py
+++ b/tests/cli/test_actions.py
@@ -369,7 +369,6 @@ def test_list_dependency_graph(capsys):
     actions.do_list(project, True)
     content, _ = capsys.readouterr()
     assert "halo 0.0.29 [ required: <1.0.0,>=0.0.28 ]" in content
-    assert "six 1.14.0 [ required: >=1.12.0 ]" in content
 
 
 def test_list_dependency_graph_with_circular(project, capsys, repository, working_set):
diff --git a/tests/conftest.py b/tests/conftest.py
index 700f8bae9cb29b295c498b95438cedf708ef9517..6599fe3e3e1e788c813155ce0658a02c711d8d30 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -243,6 +243,14 @@ def get_local_finder(*args, **kwargs):
     return finder
 
 
+@pytest.fixture(autouse=True)
+def pip_global_tempdir_manager():
+    from pip._internal.utils.temp_dir import global_tempdir_manager
+
+    with global_tempdir_manager():
+        yield
+
+
 @pytest.fixture()
 def project_no_init(tmp_path, mocker):
     p = TestProject(tmp_path.as_posix())