Update setuptools to 50.3.0

[pyup-bot]

This PR updates setuptools from 50.0.0 to 50.3.0.

Changelog

50.3.0

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* 2368: In distutils, restore support for monkeypatched CCompiler.spawn per pypa/distutils15.

50.2.0

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* 2355: When pip is imported as part of a build, leave distutils patched.
* 2380: There are some setuptools specific changes in the
`setuptools.command.bdist_rpm` module that are no longer needed, because
they are part of the `bdist_rpm` module in distutils in Python
3.5.0. Therefore, code was removed from `setuptools.command.bdist_rpm`.

50.1.0

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* 2350: Setuptools reverts using the included distutils by default. Platform maintainers and system integrators and others are *strongly* encouraged to set ``SETUPTOOLS_USE_DISTUTILS=local`` to help identify and work through the reported issues with distutils adoption, mainly to file issues and pull requests with pypa/distutils such that distutils performs as needed across every supported environment.

50.0.3

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* 2363: Restore link_libpython support on Python 3.7 and earlier (see pypa/distutils9).

50.0.2

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* 2352: In distutils hack, use absolute import rather than relative to avoid bpo-30876.

50.0.1

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* 2357: Restored Python 3.5 support in distutils.util for missing `subprocess._optim_args_from_interpreter_flags`.
* 2358: Restored AIX support on Python 3.8 and earlier.
* 2361: Add Python 3.10 support to _distutils_hack. Get the 'Loader' abstract class
from importlib.abc rather than importlib.util.abc (alias removed in Python
3.10).

Links

• PyPI: https://pypi.org/project/setuptools
• Changelog: https://pyup.io/changelogs/setuptools/
• Repo: https://github.com/pypa/setuptools

Resolves #2283