Update pip to 21.0

[pyup-bot]

This PR updates pip from 20.3.3 to 21.0.

Changelog

21.0

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

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

- Drop support for Python 2. (`6148 <https://github.com/pypa/pip/issues/6148>`_)
- Remove support for legacy wheel cache entries that were created with pip
versions older than 20.0. (`7502 <https://github.com/pypa/pip/issues/7502>`_)
- Remove support for VCS pseudo URLs editable requirements. It was emitting
deprecation warning since version 20.0. (`7554 <https://github.com/pypa/pip/issues/7554>`_)
- Modernise the codebase after Python 2. (`8802 <https://github.com/pypa/pip/issues/8802>`_)
- Drop support for Python 3.5. (`9189 <https://github.com/pypa/pip/issues/9189>`_)
- Remove the VCS export feature that was used only with editable VCS
requirements and had correctness issues. (`9338 <https://github.com/pypa/pip/issues/9338>`_)

Features
--------

- Add ``--ignore-requires-python`` support to pip download. (`1884 <https://github.com/pypa/pip/issues/1884>`_)
- New resolver: Error message shown when a wheel contains inconsistent metadata
is made more helpful by including both values from the file name and internal
metadata. (`9186 <https://github.com/pypa/pip/issues/9186>`_)

Bug Fixes
---------

- Fix a regression that made ``pip wheel`` do a VCS export instead of a VCS clone
for editable requirements. This broke VCS requirements that need the VCS
information to build correctly. (`9273 <https://github.com/pypa/pip/issues/9273>`_)
- Fix ``pip download`` of editable VCS requirements that need VCS information
to build correctly. (`9337 <https://github.com/pypa/pip/issues/9337>`_)

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

- Upgrade msgpack to 1.0.2.
- Upgrade requests to 2.25.1.

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

- Render the unreleased pip version change notes on the news page in docs. (`9172 <https://github.com/pypa/pip/issues/9172>`_)
- Fix broken email link in docs feedback banners. (`9343 <https://github.com/pypa/pip/issues/9343>`_)


.. note

 You should *NOT* be adding new change log entries to this file, this
 file is managed by towncrier. You *may* edit previous change logs to
 fix problems like typo corrections or such.

 To add a new change log entry, please see
     https://pip.pypa.io/en/latest/development/contributing/#news-entries

.. towncrier release notes start

20.3.4

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

Features
--------

- ``pip wheel`` now verifies the built wheel contains valid metadata, and can be
installed by a subsequent ``pip install``. This can be disabled with
``--no-verify``. (`9206 <https://github.com/pypa/pip/issues/9206>`_)
- Improve presentation of XMLRPC errors in pip search. (`9315 <https://github.com/pypa/pip/issues/9315>`_)

Bug Fixes
---------

- Fixed hanging VCS subprocess calls when the VCS outputs a large amount of data
on stderr. Restored logging of VCS errors that was inadvertently removed in pip
20.2. (`8876 <https://github.com/pypa/pip/issues/8876>`_)
- Fix error when an existing incompatibility is unable to be applied to a backtracked state. (`9180 <https://github.com/pypa/pip/issues/9180>`_)
- New resolver: Discard a faulty distribution, instead of quitting outright.
This implementation is taken from 20.2.2, with a fix that always makes the
resolver iterate through candidates from indexes lazily, to avoid downloading
candidates we do not need. (`9203 <https://github.com/pypa/pip/issues/9203>`_)
- New resolver: Discard a source distribution if it fails to generate metadata,
instead of quitting outright. This implementation is taken from 20.2.2, with a
fix that always makes the resolver iterate through candidates from indexes
lazily, to avoid downloading candidates we do not need. (`9246 <https://github.com/pypa/pip/issues/9246>`_)

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

- Upgrade resolvelib to 0.5.4.

Links

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