Set USE_TZ explicitly to fix some failing tests

In Django's `main` branch, the default value of the `USE_TZ` setting was changed from `False` to `True`
(see django/django@602d9a3),
which caused the tests relying on timezone-naive `datetime` objects, to fail.

Example of some of the failing tests:
https://github.com/jazzband/django-simple-history/actions/runs/4108104757/jobs/7088397467.

Author: ddabble

runtests.py

@@ -135,6 +135,7 @@ def __getitem__(self, item):
         }
     ],
     DEFAULT_AUTO_FIELD="django.db.models.AutoField",
+    USE_TZ=False,
 )
 MIDDLEWARE = [
     "django.contrib.sessions.middleware.SessionMiddleware",