`mkdir -p` and `os.makedirs()` functionality. When true, ignore
FileExistsErrors. Patch by Berker Peksag.
(With minor cleanups, additional tests, doc tweaks, etc. by Barry)
Also:
* Remove some unused imports in test_pathlib.py reported by pyflakes.
* Issue #19811, #22022: test_pathlib uses support.rmtree() instead of
shutil.rmtree() to remove the temporary directory.
* Issue #19629: Fix support.rmtree(), use os.lstat() to check if the file is a
directory, not os.path.isdir()
