remove explicit references to pytz [#184132800]

[uraniumanchor]

Contributing to the Donation Tracker

• I've added tests or modified existing tests for the change.
• I've humanly end-to-end tested the change by running an instance of the tracker.

Issue from Pivotal Tracker

https://www.pivotaltracker.com/story/show/184132800

Description of the Change

For reasons I can't entirely remember, there are some odd conflicts we were running into with tzdata and such that required pinning old versions, which are also blocking some PRs from passing via autobumps.

zoneinfo is the official way to handle timezones, but it's only supported in Python 3.9 or higher. 3.8 has a backports package available.

I took the time to remove all explicit references to pytz (and by extension, tzdata), and cleaned up some time processing code at the same... time.

Verification Process

Most of it was fairly mechanical and was covered by the tests, I did make sure that sending a donation still worked since there was some rather dubious explicit filling in of the timestamp when the default value should have been just fine.

[faultyserver]

Always glad to be able to get rid of explicit timezone references, and dropping some dependencies and removing extra utils is always great, too