Defer finalize options

Avoid loading working set during `Distribution.finalize_options` prior to invoking `_install_setup_requires`, broken since v42.0.0. Fixes #2158.

DOC: Update mailing list link

Per https://groups.google.com/d/msg/pypa-dev/rUNsfIbruHM/LCEx-CB5AgAJ
the pypa-dev Google Group is now decommissioned.
Pointing to distutils-sig instead.
Signed-off-by: Sumana Harihareswara <sh@changeset.nyc>

In wheel-to-egg conversion, use simple pkg_resources-style namespace declaration for packages that declare namespace_packages. Fixes #2070.

DOC: fix link missing >

The `decalrative config`_ link present higher in the document is also
unresolved, I'm not quite sure what the intended target was.

Crash under Python 2

Update distutils patch to monkeypatch all paths from sys.path to ensure that distutils is never imported except from the same path as setuptools. Assert that 'distutils' is not already in sys.modules.

Give a less confusing example for requirements parsing

Run docs build as part of CI

Moved distutils' doc for setup-keywords into our doc to document all supported keywords

Force fail on Python 2. When doing so, emit an error that directs users to the latest guidance. Fixes #2094.