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).

Supply a copy of _optim_args_from_interpreter_flags for Python 3.5 compatibility. Ref pypa/setuptools#2357.

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