Remove pytz from dependencies

[browniebroke]

Description

Remove pytz from the dependencies.

Checklist:

• I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates

Rationale

Django 4.0 introduced support for zoneinfo from the Python standard library (added in Python 3.9), and using pytz is deprecated.

Users can keep using the deprecated library until Django 5.0 with the USE_DEPRECATED_PYTZ setting, but it's not set in the project, which means it's disabled, which means that pytz is not needed.