add pypy schemas to command/install.py

Add support for PyPy

In test_unixcompiler.test_osx*, also patch sysconfig.get_config_vars following the patterns of prior implementations.

Acknowledge and ignore warning about TestDistribution (it's a "test" distribution, not a "test of distributions").

Mark test_venv to be skipped when running under a virtualenv as virtualenv monkey patches distutils.

Replace distutils rather than requiring it to be present in advanec. Instead of crashing, issue a warning when Setuptools is replacing distutils.

Co-authored-by: Steve Dower <steve.dower@microsoft.com>

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.