Update pip to 20.3.3

[pyup-bot]

This PR updates pip from 20.3.1 to 20.3.3.

Changelog

20.3.3

===================

Bug Fixes
---------

- Revert "Skip candidate not providing valid metadata", as that caused pip to be overeager about downloading from the package index. (`9264 <https://github.com/pypa/pip/issues/9264>`_)

20.3.2

===================

Features
--------

- New resolver: Resolve direct and pinned (``==`` or ``===``) requirements first
to improve resolver performance. (`9185 <https://github.com/pypa/pip/issues/9185>`_)
- Add a mechanism to delay resolving certain packages, and use it for setuptools. (`9249 <https://github.com/pypa/pip/issues/9249>`_)

Bug Fixes
---------

- New resolver: The "Requirement already satisfied" log is not printed only once
for each package during resolution. (`9117 <https://github.com/pypa/pip/issues/9117>`_)
- Fix crash when logic for redacting authentication information from URLs
in ``--help`` is given a list of strings, instead of a single string. (`9191 <https://github.com/pypa/pip/issues/9191>`_)
- New resolver: Correctly implement PEP 592. Do not return yanked versions from
an index, unless the version range can only be satisfied by yanked candidates. (`9203 <https://github.com/pypa/pip/issues/9203>`_)
- New resolver: Make constraints also apply to package variants with extras, so
the resolver correctly avoids backtracking on them. (`9232 <https://github.com/pypa/pip/issues/9232>`_)
- New resolver: Discard a candidate if it fails to provide metadata from source,
or if the provided metadata is inconsistent, instead of quitting outright. (`9246 <https://github.com/pypa/pip/issues/9246>`_)

Vendored Libraries
------------------

- Update vendoring to 20.8

Improved Documentation
----------------------

- Update documentation to reflect that pip still uses legacy resolver by default in Python 2 environments. (`9269 <https://github.com/pypa/pip/issues/9269>`_)

Links

• PyPI: https://pypi.org/project/pip
• Changelog: https://pyup.io/changelogs/pip/
• Homepage: https://pip.pypa.io/

[duckinator]

bors r+

[bors]

Build succeeded:

• Linux container:python:3.6-slim
• Linux container:python:3.8-slim
• Lint
• Windows PYTHON:3.7.9
• Linux container:python:3.7-slim
• Windows PYTHON:3.9.0
• FreeBSD LANG:C.UTF-8 LC_ALL:C.UTF-8 PYTHON:3.7
• FreeBSD LANG:en_US.UTF-8 LC_ALL:en_US.UTF-8 PYTHON:3.6
• Windows PYTHON:3.6.8
• macOS
• FreeBSD LANG:C.UTF-8 LC_ALL:C.UTF-8 PYTHON:3.8
• Windows PYTHON:3.8.6