Move datatracker off of python 3.9

[rjsparks]

Move to 3.12 if possible.
Prepare for moving forward following https://devguide.python.org/versions/ in mind.

[jennifer-richards]

Depending on timing and where we are with Django, it might be possible to jump to 3.13. That'll require Django 5.1, though, so we shouldn't hold this up for that.

[microamp]

3.13 has been released.

https://code.djangoproject.com/ticket/34900 (a PR has been created, currently pending)

[jennifer-richards]

I did a quick exploration of building a dev image against Python 3.12. The image built and seemed to resolve its dependencies without adjustments, which is encouraging. As noted below, it could not actually start up Django, though, so there may be demons waiting in the shadows.

The immediate blocker was the removal of the (long-)deprecated smtpd library in 3.12. ("Why did it have to be email?!" @rjsparks asked rhetorically; reports that he was in full Indiana Jones regalia are unconfirmed 🐍 ) This is only used in our dev/test mode SMTP server as far as I can tell. The fix is to migrate from smtpd to aiosmtpd, which is likely fairly straightforward.