Remove dependency on dummy_threading (to solve a bootstrap problem).
Merge
Issue #17331:  Use isidentifier() instead of isalnum() to check for valid identifiers.
Merge
#17333: test_imaplib now works with unittest test discovery.  Patch by Zachary Ware.
#17333: merge with 3.3.
#17334: test_index now works with unittest test discovery.  Patch by Zachary Ware.
#17334: merge with 3.3.