Fixed _pickle.Unpickler to handle empty persistent IDs correctly.
Issue #19789: Clarified documentation for logging.disable.
Issue #19845: Updated the Compiling Python on Windows docs.
Issue #6477: Added support for pickling the types of built-in singletons.
Issue #6477: Keep PyNotImplemented_Type and PyNone_Type private.
Fixes Issue #15798 - subprocess.Popen() no longer fails if file
descriptor 0, 1 or 2 is closed.