Fix exception causes all over the codebase

Update vendor packaging in pkg_resources to v19.2

Remove site.py hack

Remove superfluous RequirementParseError

Adopt distutils

Move distutils to setuptools._distutils

Ensure the module is named 'distutils'. Avoids errors when distutils.log and setuptools._distutils.log are two separate modules with separate state.

Fix typo in path to vendored.txt

add pypy schemas to command/install.py

Improve coverage configuration

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.