Allow for Union[None, List[datetime.datetime]] values when printing user table in weekly mail logs

[begincalendar]

Effect

I received the following in my weekly mail log email (Cron root@mail (cd /root/mailinabox && management/daily_tasks.sh)):

Traceback (most recent call last):
  File "/root/mailinabox/management/mail_log.py", line 869, in <module>
    scan_mail_log(env_vars)
  File "/root/mailinabox/management/mail_log.py", line 146, in scan_mail_log
    print_user_table(
  File "/root/mailinabox/management/mail_log.py", line 682, in print_user_table
    if None not in {latest, earliest}:
TypeError: unhashable type: 'list'

Cause

This was overzealous in that it didn't accommodate for the fact that earliest and latest have the type Union[None, List[datetime.dateime]].

I think this PR supersedes #2376.

[kimusan]

Good solution.

[dms00]

Nice fix. Is there a comment we can add to those lines so the linter won't try to change them again in the future?

[dms00]

Looks like # noqa PLR6201 should do the trick.

[benschumacher]

I just encountered this issue, as well. Thanks for the fix!