Resolve flake8 <-> black conflicts

.circleci/config.yml

@@ -26,7 +26,7 @@ jobs:
             python3 -m venv venv
             . venv/bin/activate
             pip install --upgrade pip
-            pip install -r .circleci/requirements.txt
+            pip install -r requirements.txt
 
       - save_cache:
           paths:

.circleci/requirements.txt

-black==18.9b0
-flake8==3.6.0

pyproject.toml

 [tool.black]
 line-length = 79
-py36 = true
+target-version = ["py36"]
 exclude = '''
 /(
   \.git

requirements.txt

+black==19.3b0
+flake8==3.7.7

setup.cfg

 [flake8]
 exclude = .git,venv
+# Handle formatting conflicts between Black and Flake8:
+# https://github.com/python/black/issues/280 (E203)
+# https://github.com/python/black/issues/43 (W503)
+ignore = E203,W503
 
 [pycodestyle]
 exclude = .git,venv