Backported test for the open of non-existent tarfile.
#20236: Fix sphinx markup.
Fix typo.  Pointed out by Finn Ellis on docs@.
complain when nbytes > buflen to fix possible buffer overflow (closes #20246)
correct defaultdict signature in docstring (closes #20250)

Patch from Andrew Barnert.
remove overly strict assertion (closes #20251)
add test for #20251