Notifier crashes due to incompatible datetimes being subtracted

[fniessink]

notifier_1       |   Traceback (most recent call last):
notifier_1       |   File "/home/notifier/quality_time_notifier.py", line 113, in <module>
notifier_1       |     asyncio.run(notify(logging.INFO))  # pragma: no cover
notifier_1       |   File "/usr/local/lib/python3.9/asyncio/runners.py", line 44, in run
notifier_1       |     return loop.run_until_complete(main)
notifier_1       |   File "/usr/local/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
notifier_1       |     return future.result()
notifier_1       |   File "/home/notifier/quality_time_notifier.py", line 41, in notify
notifier_1       |     notifications = notification_finder.get_notifications(json, most_recent_measurement_seen)
notifier_1       |   File "/home/notifier/strategies/notification_strategy.py", line 24, in get_notifications
notifier_1       |     notable = self.get_notification(metric, metric_uuid, most_recent_measurement_seen)
notifier_1       |   File "/home/notifier/strategies/notification_strategy.py", line 37, in get_notification
notifier_1       |     and self.long_unchanged_status(metric, metric_uuid, most_recent_measurement_seen)
notifier_1       |   File "/home/notifier/strategies/notification_strategy.py", line 61, in long_unchanged_status
notifier_1       |     difference = datetime_most_recent_measurement_seen - status_start
notifier_1       | TypeError: can't subtract offset-naive and offset-aware datetimes
quality-time_notifier_1 exited with code 1