Bump pip from 19.2 to 21.1 in /requirements

[dependabot]

Bumps pip from 19.2 to 21.1.

Changelog

Sourced from pip's changelog.

21.1 (2021-04-24)

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) <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) <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) <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) <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) <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) <https://github.com/pypa/pip/issues/9774>_)

Bug Fixes

• Fixed --target to work with --editable installs. ([#4390](https://github.com/pypa/pip/issues/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) <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) <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) <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) <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) <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) <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) <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) <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) <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) <https://github.com/pypa/pip/issues/9409>_)
• Fix incorrect reporting on Requires-Python conflicts. ([#9541](https://github.com/pypa/pip/issues/9541) <https://github.com/pypa/pip/issues/9541>_)

... (truncated)

Commits

• 2b2a268 Bump for release
• ea761a6 Update AUTHORS.txt
• 2edd3fd Postpone a deprecation to 21.2
• 3cccfbf Rename mislabeled news fragment
• 21cd124 Fix NEWS.rst placeholder position
• e46bdda Merge pull request #9827 from pradyunsg/fix-git-improper-tag-handling
• 0e4938d 📰
• ca832b2 Don't split git references on unicode separators
• 1320bac Merge pull request #9814 from pradyunsg/revamp-ci-apr-2021-v2
• e9cc23f Skip checks on PRs only
• Additional commits viewable in compare view

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