#17315: unlink a file that test_posixpath was leaving around.
#17315: merge with 3.2.
Fix markup in unittest doc.
Merge markup fixes in unittest doc from 3.2.
Remove dependency on dummy_threading (to solve a bootstrap problem).
Issue #17331:  Use isidentifier() instead of isalnum() to check for valid identifiers.
#17333: test_imaplib now works with unittest test discovery.  Patch by Zachary Ware.
#17334: test_index now works with unittest test discovery.  Patch by Zachary Ware.