Update pip to 21.1

[pyup-bot]

This PR updates pip from 21.0.1 to 21.1.

Changelog

21.1

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

Process
-------

- Start installation scheme migration from ``distutils`` to ``sysconfig``. A
warning is implemented to detect differences between the two implementations to
encourage user reports, so we can avoid breakages before they happen.

Features
--------

- Add the ability for the new resolver to process URL constraints. (`8253 <https://github.com/pypa/pip/issues/8253>`_)
- Add a feature ``--use-feature=in-tree-build`` to build local projects in-place
when installing. This is expected to become the default behavior in pip 21.3;
see `Installing from local packages <https://pip.pypa.io/en/stable/user_guide/#installing-from-local-packages>`_
for more information. (`9091 <https://github.com/pypa/pip/issues/9091>`_)
- Bring back the "(from versions: ...)" message, that was shown on resolution failures. (`9139 <https://github.com/pypa/pip/issues/9139>`_)
- Add support for editable installs for project with only setup.cfg files. (`9547 <https://github.com/pypa/pip/issues/9547>`_)
- Improve performance when picking the best file from indexes during ``pip install``. (`9748 <https://github.com/pypa/pip/issues/9748>`_)
- Warn instead of erroring out when doing a PEP 517 build in presence of
``--build-option``. Warn when doing a PEP 517 build in presence of
``--global-option``. (`9774 <https://github.com/pypa/pip/issues/9774>`_)

Bug Fixes
---------

- Fixed ``--target`` to work with ``--editable`` installs. (`4390 <https://github.com/pypa/pip/issues/4390>`_)
- Add a warning, discouraging the usage of pip as root, outside a virtual environment. (`6409 <https://github.com/pypa/pip/issues/6409>`_)
- Ignore ``.dist-info`` directories if the stem is not a valid Python distribution
name, so they don't show up in e.g. ``pip freeze``. (`7269 <https://github.com/pypa/pip/issues/7269>`_)
- Only query the keyring for URLs that actually trigger error 401.
This prevents an unnecessary keyring unlock prompt on every pip install
invocation (even with default index URL which is not password protected). (`8090 <https://github.com/pypa/pip/issues/8090>`_)
- Prevent packages already-installed alongside with pip to be injected into an
isolated build environment during build-time dependency population. (`8214 <https://github.com/pypa/pip/issues/8214>`_)
- Fix ``pip freeze`` permission denied error in order to display an understandable error message and offer solutions. (`8418 <https://github.com/pypa/pip/issues/8418>`_)
- Correctly uninstall script files (from setuptools' ``scripts`` argument), when installed with ``--user``. (`8733 <https://github.com/pypa/pip/issues/8733>`_)
- New resolver: When a requirement is requested both via a direct URL
(``req  URL``) and via version specifier with extras (``req[extra]``), the
resolver will now be able to use the URL to correctly resolve the requirement
with extras. (`8785 <https://github.com/pypa/pip/issues/8785>`_)
- New resolver: Show relevant entries from user-supplied constraint files in the
error message to improve debuggability. (`9300 <https://github.com/pypa/pip/issues/9300>`_)
- Avoid parsing version to make the version check more robust against lousily
debundled downstream distributions. (`9348 <https://github.com/pypa/pip/issues/9348>`_)
- ``--user`` is no longer suggested incorrectly when pip fails with a permission
error in a virtual environment. (`9409 <https://github.com/pypa/pip/issues/9409>`_)
- Fix incorrect reporting on ``Requires-Python`` conflicts. (`9541 <https://github.com/pypa/pip/issues/9541>`_)
- Make wheel compatibility tag preferences more important than the build tag (`9565 <https://github.com/pypa/pip/issues/9565>`_)
- Fix pip to work with warnings converted to errors. (`9779 <https://github.com/pypa/pip/issues/9779>`_)
- **SECURITY**: Stop splitting on unicode separators in git references,
which could be maliciously used to install a different revision on the
repository. (`9827 <https://github.com/pypa/pip/issues/9827>`_)

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

- Update urllib3 to 1.26.4 to fix CVE-2021-28363
- Remove contextlib2.
- Upgrade idna to 3.1
- Upgrade pep517 to 0.10.0
- Upgrade vendored resolvelib to 0.7.0.
- Upgrade tenacity to 7.0.0

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

- Update "setuptools extras" link to match upstream. (`4822829F-6A45-4202-87BA-A80482DF6D4E <https://github.com/pypa/pip/issues/4822829F-6A45-4202-87BA-A80482DF6D4E>`_)
- Improve SSL Certificate Verification docs and ``--cert`` help text. (`6720 <https://github.com/pypa/pip/issues/6720>`_)
- Add a section in the documentation to suggest solutions to the ``pip freeze`` permission denied issue. (`8418 <https://github.com/pypa/pip/issues/8418>`_)
- Add warning about ``--extra-index-url`` and dependency confusion (`9647 <https://github.com/pypa/pip/issues/9647>`_)
- Describe ``--upgrade-strategy`` and direct requirements explicitly; add a brief
example. (`9692 <https://github.com/pypa/pip/issues/9692>`_)

Links

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

[pyup-bot]

Closing this in favor of #2