Re-enable distutils patch by default.

Resolving error text issue (#2302)

# Conflicts:
#	setuptools/msvc.py

Remove legacy Python 2 code

Replace hard-coded tox working dir with the environment variable, honoring non-standard working directories.

Stricter egg detection

Don't install setup_requires when run as a PEP-517 backend.

Update packaging to 20.4

docs: Suggestion - Provide consistency in the description of setup.cfg

Improve safe_version documentation

Change load_module to exec_module

Remove superfluous 'v' in CHANGES.rst

This was an artifact of the fact that I did not use `tox -e finalize`
to do the latest release in order to make a patch release.

Fix issue with distutils warning