Update setuptools to 51.1.0.post20201221

[pyup-bot]

This PR updates setuptools from 51.0.0 to 51.1.0.post20201221.

Changelog

51.1.0

-------


Changes
^^^^^^^
* 2486: Project adopts jaraco/skeleton for shared package maintenance.

Misc
^^^^
* 2477: Restore inclusion of rst files in sdist.
* 2484: Setuptools has replaced the master branch with the main branch.
* 2485: Fixed failing test when pip 20.3+ is present.
-- by :user:`yan12125`
* 2487: Fix tests with pytest 6.2
-- by :user:`yan12125`

Links

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

[Harmon758]

Superseded by #2573 (pypa/setuptools#2500)