Drop `pytz` dependency

[Archmonger]

Discussed in #590

^{Originally posted by johnthagen June 15, 2025}
We currently have a dependency on pytz:

django-dbbackup/requirements.txt

Line 2 in 91e3611

pytz

We should port away from this like core Django did.

• https://docs.djangoproject.com/en/5.2/releases/4.0/#zoneinfo-default-timezone-implementation
• https://stackoverflow.com/questions/76583100/is-pytz-deprecated-now-or-in-the-future-in-python