Fix timezone aware comparison.

[mush]

django_cron.FailedRunsNotificationCronJob is throwing the following exception with USE_TZ=True.

  File "/.venv/lib64/python3.8/site-packages/django_cron/management/commands/runcrons.py", line 82, in run_cron_with_cache_check
    manager.run(force)
  File "/.venv/lib64/python3.8/site-packages/django_cron/core.py", line 258, in run
    if self.should_run_now(force):
  File "/.venv/lib64/python3.8/site-packages/django_cron/core.py", line 138, in should_run_now
    self.previously_ran_successful_cron = CronJobLog.objects.filter(
  File "/.venv/lib64/python3.8/site-packages/django/db/models/query.py", line 949, in exclude
    return self._filter_or_exclude(True, args, kwargs)
  File "/.venv/lib64/python3.8/site-packages/django/db/models/query.py", line 961, in _filter_or_exclude
    clone._filter_or_exclude_inplace(negate, args, kwargs)
  File "/.venv/lib64/python3.8/site-packages/django/db/models/query.py", line 966, in _filter_or_exclude_inplace
    self._query.add_q(~Q(*args, **kwargs))
  File "/.venv/lib64/python3.8/site-packages/django/db/models/sql/query.py", line 1416, in add_q
    clause, _ = self._add_q(q_object, self.used_aliases)
  File "/.venv/lib64/python3.8/site-packages/django/db/models/sql/query.py", line 1435, in _add_q
    child_clause, needed_inner = self.build_filter(
  File "/.venv/lib64/python3.8/site-packages/django/db/models/sql/query.py", line 1370, in build_filter
    condition = self.build_lookup(lookups, col, value)
  File "/.venv/lib64/python3.8/site-packages/django/db/models/sql/query.py", line 1216, in build_lookup
    lookup = lookup_class(lhs, rhs)
  File "/.venv/lib64/python3.8/site-packages/django/db/models/lookups.py", line 25, in __init__
    self.rhs = self.get_prep_lookup()
  File "/.venv/lib64/python3.8/site-packages/django/db/models/lookups.py", line 77, in get_prep_lookup
    return self.lhs.output_field.get_prep_value(self.rhs)
  File "/.venv/lib64/python3.8/site-packages/django/db/models/fields/__init__.py", line 1416, in get_prep_value
    warnings.warn("DateTimeField %s received a naive datetime (%s)"
RuntimeWarning: DateTimeField CronJobLog.start_time received a naive datetime (2022-10-13 11:02:56.495028) while time zone support is active.