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 a12255d8de)
Co-authored-by: David K <dave@paddez.com>
https://bugs.python.org/issue38421
Automerge-Triggered-By: @encukou