#6916: raise a deprecation warning if using asynchat.fifo
merge heads
Issue 21635:  Fix caching in difflib.SequenceMatcher.get_matching_blocks().
merge
Issue 21786:  Clean-up test_pydoc taking taking advantage of diffing in unittest.
Issue #21823: Catch turtle.Terminator exceptions in turtledemo.
Add note to demohelp.txt about doing so.
Merge with 3.4