Issue #19902: Added list of logging levels.
Closes #19902: Merged update from 3.3.
Issue #19946: appropriately skip new multiprocessing tests

Thanks to Christian Heimes for noting the buildbot failures and
to Zachary Ware for providing the patch to make the new tests
play nice with both other platforms and unittest test discovery
Fix os.listdir(): _Py_dup() already raises an exception on error, no need to
raise a new exception