Update pip to 20.3

[pyup-bot]

This PR updates pip from 20.2.4 to 20.3.

Changelog

20.3

- Introduce a new ResolutionImpossible error, raised when pip encounters un-satisfiable dependency conflicts (`8546 <https://github.com/pypa/pip/issues/8546>`_, `8377 <https://github.com/pypa/pip/issues/8377>`_)
- Add a subcommand ``debug`` to ``pip config`` to list available configuration sources and the key-value pairs defined in them. (`6741 <https://github.com/pypa/pip/issues/6741>`_)
- Warn if index pages have unexpected content-type (`6754 <https://github.com/pypa/pip/issues/6754>`_)
- Allow specifying ``--prefer-binary`` option in a requirements file (`7693 <https://github.com/pypa/pip/issues/7693>`_)
- Generate PEP 376 REQUESTED metadata for user supplied requirements installed
by pip. (`7811 <https://github.com/pypa/pip/issues/7811>`_)
- Warn if package url is a vcs or an archive url with invalid scheme (`8128 <https://github.com/pypa/pip/issues/8128>`_)
- Parallelize network operations in ``pip list``. (`8504 <https://github.com/pypa/pip/issues/8504>`_)
- Allow the new resolver to obtain dependency information through wheels
lazily downloaded using HTTP range requests.  To enable this feature,
invoke ``pip`` with ``--use-feature=fast-deps``. (`8588 <https://github.com/pypa/pip/issues/8588>`_)
- Support ``--use-feature`` in requirements files (`8601 <https://github.com/pypa/pip/issues/8601>`_)

Bug Fixes
---------

- Use canonical package names while looking up already installed packages. (`5021 <https://github.com/pypa/pip/issues/5021>`_)
- Fix normalizing path on Windows when installing package on another logical disk. (`7625 <https://github.com/pypa/pip/issues/7625>`_)
- The VCS commands run by pip as subprocesses don't merge stdout and stderr anymore, improving the output parsing by subsequent commands. (`7968 <https://github.com/pypa/pip/issues/7968>`_)
- Correctly treat non-ASCII entry point declarations in wheels so they can be
installed on Windows. (`8342 <https://github.com/pypa/pip/issues/8342>`_)
- Update author email in config and tests to reflect decommissioning of pypa-dev list. (`8454 <https://github.com/pypa/pip/issues/8454>`_)
- Headers provided by wheels in .data directories are now correctly installed
into the user-provided locations, such as ``--prefix``, instead of the virtual
environment pip is running in. (`8521 <https://github.com/pypa/pip/issues/8521>`_)

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

- Vendored htmlib5 no longer imports deprecated xml.etree.cElementTree on Python 3.
- Upgrade appdirs to 1.4.4
- Upgrade certifi to 2020.6.20
- Upgrade distlib to 0.3.1
- Upgrade html5lib to 1.1
- Upgrade idna to 2.10
- Upgrade packaging to 20.4
- Upgrade requests to 2.24.0
- Upgrade six to 1.15.0
- Upgrade toml to 0.10.1
- Upgrade urllib3 to 1.25.9

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

- Add ``--no-input`` option to pip docs (`7688 <https://github.com/pypa/pip/issues/7688>`_)
- List of options supported in requirements file are extracted from source of truth,
instead of being maintained manually. (`7908 <https://github.com/pypa/pip/issues/7908>`_)
- Fix pip config docstring so that the subcommands render correctly in the docs (`8072 <https://github.com/pypa/pip/issues/8072>`_)
- replace links to the old pypa-dev mailing list with https://mail.python.org/mailman3/lists/distutils-sig.python.org/ (`8353 <https://github.com/pypa/pip/issues/8353>`_)
- Fix example for defining multiple values for options which support them (`8373 <https://github.com/pypa/pip/issues/8373>`_)
- Add documentation for the ResolutionImpossible error that helps the user fix dependency conflicts (`8459 <https://github.com/pypa/pip/issues/8459>`_)
- Add feature flags to docs (`8512 <https://github.com/pypa/pip/issues/8512>`_)
- Document how to install package extras from git branch and source distributions. (`8576 <https://github.com/pypa/pip/issues/8576>`_)

20.3b1

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

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

- ``pip freeze`` will stop filtering the ``pip``, ``setuptools``, ``distribute`` and ``wheel`` packages from ``pip freeze`` output in a future version.
To keep the previous behavior, users should use the new ``--exclude`` option. (`4256 <https://github.com/pypa/pip/issues/4256>`_)
- Deprecate support for Python 3.5 (`8181 <https://github.com/pypa/pip/issues/8181>`_)
- 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>`_)
- Deprecate ``--find-links`` option in ``pip freeze`` (`9069 <https://github.com/pypa/pip/issues/9069>`_)

Features
--------

- Add ``--exclude`` option to ``pip freeze`` and ``pip list`` commands to explicitly exclude packages from the output. (`4256 <https://github.com/pypa/pip/issues/4256>`_)
- Allow multiple values for --abi and --platform. (`6121 <https://github.com/pypa/pip/issues/6121>`_)
- Add option ``--format`` to subcommand ``list`` of ``pip  cache``, with ``abspath`` choice to output the full path of a wheel file. (`8355 <https://github.com/pypa/pip/issues/8355>`_)
- Improve error message friendliness when an environment has packages with
corrupted metadata. (`8676 <https://github.com/pypa/pip/issues/8676>`_)
- Make the ``setup.py install`` deprecation warning less noisy. We warn only
when ``setup.py install`` succeeded and ``setup.py bdist_wheel`` failed, as
situations where both fails are most probably irrelevant to this deprecation. (`8752 <https://github.com/pypa/pip/issues/8752>`_)
- Check the download directory for existing wheels to possibly avoid
fetching metadata when the ``fast-deps`` feature is used with
``pip wheel`` and ``pip download``. (`8804 <https://github.com/pypa/pip/issues/8804>`_)
- When installing a git URL that refers to a commit that is not available locally
after git clone, attempt to fetch it from the remote. (`8815 <https://github.com/pypa/pip/issues/8815>`_)
- Include http subdirectory in ``pip cache info`` and ``pip cache purge`` commands. (`8892 <https://github.com/pypa/pip/issues/8892>`_)
- 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>`_)
- Support Python 3.9. (`8971 <https://github.com/pypa/pip/issues/8971>`_)
- Log an informational message when backtracking takes multiple rounds on a specific package. (`8975 <https://github.com/pypa/pip/issues/8975>`_)
- Switch to the new dependency resolver by default. (`9019 <https://github.com/pypa/pip/issues/9019>`_)
- Remove the ``--build-dir`` option, as per the deprecation. (`9049 <https://github.com/pypa/pip/issues/9049>`_)

Bug Fixes
---------

- Propagate ``--extra-index-url`` from requirements file properly to session auth,
so that keyring auth will work as expected. (`8103 <https://github.com/pypa/pip/issues/8103>`_)
- Allow specifying verbosity and quiet level via configuration files
and environment variables. Previously these options were treated as
boolean values when read from there while through CLI the level can be
specified. (`8578 <https://github.com/pypa/pip/issues/8578>`_)
- Only converts Windows path to unicode on Python 2 to avoid regressions when a
POSIX environment does not configure the file system encoding correctly. (`8658 <https://github.com/pypa/pip/issues/8658>`_)
- List downloaded distributions before exiting ``pip download``
when using the new resolver to make the behavior the same as
that on the legacy resolver. (`8696 <https://github.com/pypa/pip/issues/8696>`_)
- New resolver: Pick up hash declarations in constraints files and use them to
filter available distributions. (`8792 <https://github.com/pypa/pip/issues/8792>`_)
- Avoid polluting the destination directory by resolution artifacts
when the new resolver is used for ``pip download`` or ``pip wheel``. (`8827 <https://github.com/pypa/pip/issues/8827>`_)
- 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>`_)
- Tweak the output during dependency resolution in the new resolver. (`8861 <https://github.com/pypa/pip/issues/8861>`_)
- Correctly search for installed distributions in new resolver logic in order
to not miss packages (virtualenv packages from system-wide-packages for example) (`8963 <https://github.com/pypa/pip/issues/8963>`_)
- Do not fail in pip freeze when encountering a ``direct_url.json`` metadata file
with editable=True. Render it as a non-editable ``file://`` URL until modern
editable installs are standardized and supported. (`8996 <https://github.com/pypa/pip/issues/8996>`_)

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

- Fix devendoring instructions to explicitly state that ``vendor.txt`` should not be removed.
It is mandatory for ``pip debug`` command.

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

- Add documentation for '.netrc' support. (`7231 <https://github.com/pypa/pip/issues/7231>`_)
- Add OS tabs for OS-specific commands. (`7311 <https://github.com/pypa/pip/issues/7311>`_)
- Add note and example on keyring support for index basic-auth (`8636 <https://github.com/pypa/pip/issues/8636>`_)
- Added initial UX feedback widgets to docs. (`8783 <https://github.com/pypa/pip/issues/8783>`_, `8848 <https://github.com/pypa/pip/issues/8848>`_)
- Add ux documentation (`8807 <https://github.com/pypa/pip/issues/8807>`_)
- Update user docs to reflect new resolver as default in 20.3. (`9044 <https://github.com/pypa/pip/issues/9044>`_)
- Improve migration guide to reflect changes in new resolver behavior. (`9056 <https://github.com/pypa/pip/issues/9056>`_)

Links

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

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[pyup-bot]

Closing this in favor of #197