Read timezone for Crons monitors from `celery_schedule` if existing #2971

Job	Run time
huey, python 2.7, ubuntu-20.04	1m 37s
huey, python 3.5, ubuntu-20.04	1m 27s
huey, python 3.6, ubuntu-20.04	58s
huey, python 3.7, ubuntu-20.04	1m 11s
huey, python 3.8, ubuntu-20.04	42s
huey, python 3.9, ubuntu-20.04	44s
huey, python 3.10, ubuntu-20.04	58s
huey, python 3.11, ubuntu-20.04	1m 10s
huey, python 3.12, ubuntu-20.04	54s
All huey tests passed or skipped	1s
	9m 42s

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