Update setuptools to 50.0.3

[pyup-bot]

This PR updates setuptools from 50.0.0 to 50.0.3.

Changelog

50.0.3

-------

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

50.0.2

-------

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

50.0.1

-------

* 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

[coveralls]

Coverage remained the same at 86.168% when pulling 7cc76df on pyup-update-setuptools-50.0.0-to-50.0.3 into a8efce0 on master.

[pyup-bot]

Closing this in favor of #253