Update pip to 20.2.4

[pyup-bot]

This PR updates pip from 20.2.3 to 20.2.4.

Changelog

20.2.4

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

Deprecations and Removals
-------------------------

- Document that certain removals can be fast tracked. (`8417 <https://github.com/pypa/pip/issues/8417>`_)
- Document that Python versions are generally supported until PyPI usage falls below 5%. (`8927 <https://github.com/pypa/pip/issues/8927>`_)

Features
--------

- New resolver: Avoid accessing indexes when the installed candidate is preferred
and considered good enough. (`8023 <https://github.com/pypa/pip/issues/8023>`_)
- Improve error message friendliness when an environment has packages with
corrupted metadata. (`8676 <https://github.com/pypa/pip/issues/8676>`_)
- Cache package listings on index packages so they are guarenteed to stay stable
during a pip command session. This also improves performance when a index page
is accessed multiple times during the command session. (`8905 <https://github.com/pypa/pip/issues/8905>`_)
- New resolver: Tweak resolution logic to improve user experience when
user-supplied requirements conflict. (`8924 <https://github.com/pypa/pip/issues/8924>`_)

Bug Fixes
---------

- New resolver: Correctly respect ``Requires-Python`` metadata to reject
incompatible packages in ``--no-deps`` mode. (`8758 <https://github.com/pypa/pip/issues/8758>`_)
- New resolver: Pick up hash declarations in constraints files and use them to
filter available distributions. (`8792 <https://github.com/pypa/pip/issues/8792>`_)
- New resolver: If a package appears multiple times in user specification with
different ``--hash`` options, only hashes that present in all specifications
should be allowed. (`8839 <https://github.com/pypa/pip/issues/8839>`_)

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

- Add ux documentation (`8807 <https://github.com/pypa/pip/issues/8807>`_)

Links

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

[duckinator]

bors r+

[bors]

Build succeeded:

• FreeBSD LANG:C.UTF-8 LC_ALL:C.UTF-8 PYTHON:3.7
• FreeBSD LANG:C.UTF-8 LC_ALL:C.UTF-8 PYTHON:3.8
• FreeBSD LANG:en_US.UTF-8 LC_ALL:en_US.UTF-8 PYTHON:3.6
• Lint
• Linux container:python:3.6-slim
• Linux container:python:3.7-slim
• Linux container:python:3.8-slim
• macOS
• Windows container:python:3.6-windowsservercore-1809
• Windows container:python:3.7-windowsservercore-1809