- Issue #21539: Add a *exists_ok* argument to `Pathlib.mkdir()` to mimic

- Issue #21539: Add a *exists_ok* argument to `Pathlib.mkdir()` to mimic
  `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.