Migrate from pytz to zoneinfo

[rtibbles]

Observed behavior

We have previously used the pytz library for timezone handling - however, since Python 3, the zoneinfo standard library interface has been available.

Expected behavior

We should follow the migration guide here: https://pytz-deprecation-shim.readthedocs.io/en/latest/migration.html to migrate from pytz to the zoneinfo standard library module.

All instances of pytz in the codebase should be replaced by equivalent zoneinfo imports/methods.

Finally, pytz should be removed as a dependency from requirements/base.txt.