Restore pythonlib support on older Pythons.

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

Bugfix/tests for msvc spawn

In _msvccompiler.MSVCCompiler.spawn, use correct capitalization for PATH environment variable. Fixes failing test and fixes pypa/setuptools#2257.

In CCompiler, allow keyword arguments to be passed to spawn calls. Ref pypa/setuptools#2257 and pypa/distutils#5.

In TestSpawn.test_concurrent_safe, use CheckThread to ensure that the spawn call does not simply fail to execute. Ref pypa/setuptools#2257.

Suppress ImportError for winreg as the module is only available on some platforms. Allows unit testing of module on non-Windows platforms.

Allow spawn to accept environment. Avoid monkey-patching global state.

Allow spawn to accept environment. Avoid monkey-patching global state. Closes pypa/setuptools#2212 and closes pypa/distutils#5.

Automerge-Triggered-By: @jaraco

Replace OSError with DistutilsExecError. Fixes pypa/distutils#3 and pypa/setuptools#2228 and bpo-41207.

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.