Merge with pypa/distutils@6852b20

[jaraco]

• Use path.Path for changing the cwd temporarily.
• Remove needless assert renderings. Any decent test runner will provide this detail.
• Remove reference to duplicate issue.
• Refactor _is_python_source_dir using pathlib.
• Use 'samefile' to check for same file, because they may not be the same path. Fixes 3.4 release fails tests #180.
• Split test into two tests.
• Use pathlib
• Remove special case for TestCore.write_setup.
• Replace TESTFN with temp_file fixture.
• Remove 'cleanup_testfn', unused.
• Remove distracting comments
• In test_sysconfig, prefer fixtures to TESTFN.
• Remove cleanup_testfn, no longer used.
• Replace TESTFN with fixture.
• Remove empty logic branch.
• Use path.Path for changing the cwd temporarily.
• Remove reliance on TESTFN in test_dist
• Remove TESTFN from py38compat, no longer needed.
• xfail srcdir_simple on Windows
• Remove reliance on os_helper in test_build_ext
• Remove reliance on change_cwd
• Add compatibility shim for subprocess on Python 3.7 on Windows
• Extract _save_cwd for saving the current working directory.
• Prefer a temp_cwd fixture to reduce indentation.
• Remove reliance on create_empty_file
• Add test for PermissionError. Ref [BUG] Change to pathlib causes Permission Errors when checking for config files distutils#181.
• Use os.path.isfile for checking if path.is_file, suppresses exceptions. Fixes [BUG] Change to pathlib causes Permission Errors when checking for config files distutils#181.

Summary of changes

Closes

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/.
  (See documentation for details)