Declare USE_TZ in test settings

[adamchainz]

Fix this warning seen when running the tests on Django 4.0+:

$ tox -e py310-django40
...
===== warnings summary =====
.tox/py310-django40/lib/python3.10/site-packages/django/conf/__init__.py:206
  /.../site-packages/django/conf/__init__.py:206: RemovedInDjango50Warning: The default value of USE_TZ will change from False to True in Django 5.0. Set USE_TZ to False in your project settings if you want to keep the current default behavior.
...
===== 177 passed, 2 warnings in 1.33s =====
_____ summary _____
  py310-django40: commands succeeded
  congratulations :)

As per this release note: https://docs.djangoproject.com/en/4.0/releases/4.0/#time-zone-support