[3.8] bpo-38421: Update email.utils documentation (GH-16678) (GH-17122)

[3.8] bpo-38421: Update email.utils documentation (GH-16678) (GH-17122)

Updates documentation around email.utils.parsedate_tz().

Currently, the documentation specifies that when a string without a is timezone passed to parsedate_tz(), the last tuple is returned as ```None```.

This is no longer true since Python 3.3

https://bugs.python.org/issue38421
(cherry picked from commit a12255d8def0c82560545e66c1be981a447751c3)

Co-authored-by: David K <dave@paddez.com>
https://bugs.python.org/issue38421

Automerge-Triggered-By: @encukou