Remove the pytz requirement

Django 4.0 made zoneinfo the default implementation for timezones, rather than pytz, and a USE_DEPRECATED_PYTZ setting was added to continue using pytz if we chose to.

DRF still depended on pytz for a while and only dropped it in 3.15, when they were forced to in order to support Django 5.0. So we can now remove it too.

Closes !2111 (closed)