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.

Use new test.support helper submodules in tests:

* distutils tests
* test_buffer
* test_compile
* test_filecmp
* test_fileinput
* test_readline
* test_smtpnet
* test_structmembers
* test_tools

Fix test_copyreg when numpy is installed: test.pickletester now
saves/restores warnings.filters when importing numpy, to ignore
filters installed by numpy.

Add the save_restore_warnings_filters() function to the
test.support.warnings_helper module.

- Fix upload test on systems that blocks MD5
- Add SHA2-256 and Blake2b-256 digests based on new Warehous and twine
  specs.
Signed-off-by: Christian Heimes <christian@python.org>

distutils.tests now saves/restores warnings filters to leave them
unchanged. Importing tests imports docutils which imports
pkg_resources which adds a warnings filter.