#19890: fix typo in multiprocessing docs.  Patch by Mike Short.
#19890: merge with 3.3.
give non-iterable TypeError a message (closes #20507)
merge 3.3 (#20507)
add missing test assertion (closes #20080)

Patch by Vajrasky Kok.
merge 3.3 (#20080)
#20237: make a revision pass over the XML vulnerabilities section
merge 3.3
Clarify versionchanged sentence.  Closes #20497.
Merge from 3.3
#16728: Mention collections.abc.Sequence in 'sequence' glossary entry
#20241: use correct RFC number
Merge from 3.3