Now that Python 2 is not supported, the bytes/str boundaries are more
clear and explicit. Using six.ensure_* methods for backwards
compatibility is no longer necessary as the types are known and verified
using mypy.

One exception is tests/lib/wheel.py which allows tests to pass test
setup data as either bytes or str.

The module operations.install.wheel also remains untouched as it is
especially delicate to bytes/str mixups and the current version is
working.

Clean up urllib imports

Use unified OSError and its subclasses

When the new resolver needs to upgrade a package, it puts the
already-installed package in the middle of the candidate list obtained
from indexes. But when doing it, the candidate list is eagerly consumed,
causing pip to download all candidates.

Skip candidate not providing valid metadata

This reverts commit 7165ab8c.

Since Python 3.3, the following classes have merged into OSError. They
remain as aliases for backward compatibility.

- EnvironmentError
- IOError
- WindowsError

https://docs.python.org/3/library/exceptions.html#OSError

Python 3 also has subclasses of OSError to help identify more specific
errors. For example, FileNotFoundError. This allows simplifying some
except blocks.

Use "yield from" syntax

Available since Python 3.3.

https://docs.python.org/3/whatsnew/3.3.html#pep-380

Add a preview for the "next release" change notes

Replace io.open() and codecs.open() with builtin open()

Use f-strings for simple string formatting

Harmonize type signature of VersionControl.get_remote_url() subclasses

Remove "universal" from "bdist_wheel" configuration

Fix broken email link in docs feedback banners

Remove object from class definitions

Remove Python 2 compatibility shim for urlparse.uses_fragment

Replace vendored ipaddress with stdlib

Remove unnecessary str() call around str literal

Remove Python 2 compatibility shim for json.JSONDecodeError

Replace utils.compat.lru_cache with stdlib functools.lru_cache

Unnecessary since dropping Python 2.

JSONDecodeError has been available since Python 3.5.

https://docs.python.org/3/library/json.html#json.JSONDecodeError

Use pyupgrade to convert simple string formatting to use f-string
syntax. pyupgrade is intentionally timid and will not create an f-string
if it would make the expression longer or if the substitution parameters
are anything but simple names or dotted names.

In Python 3, these are functionally equivalent and share the same
feature set.

Unnecessary since dropping Python 2 support. In Python 3, all classes
are new style classes.

In the base class, the signature is defined as:

    type: (str) -> str

Further, the docstring says:

    Raises RemoteNotFoundError if the repository does not have a remote
    url configured.

However, some subclasses were returning None instead of raising
RemoteNotFoundError. This violated the type signature and forced calling
code to handle multiple error paradigms.

Now, all subclasses implement the base's signature.

This allowed simplifying some call sites as they can assume None will
not be returned.

This mismatch was noticed while trying to remove "mypy:
disallow-untyped-defs=False" comments.

This attribute (now urllib.parse.uses_fragment) is unused by the stdlib
and remains only for backwards compatibility. See the comment:

https://github.com/python/cpython/blob/v3.6.0/Lib/urllib/parse.py#L55-L63

Use short Python3 super() syntax

Drop u prefix from str literals

The stdlib version has been available since Python 3.2.

The vendored copy is unnecessary since dropping Python 2 support. The
module has been available in the stdlib since Python 3.3.