Fix timezone parse for non-UTC timezone

Fix https://github.com/GerevAI/gerev/issues/95

>>> datetime.datetime.strptime('2017-12-05T15:00:24.972+08:00', '%Y-%m-%dT%H:%M:%S.%f%z')
datetime.datetime(2017, 12, 5, 15, 0, 24, 972000, tzinfo=datetime.timezone(datetime.timedelta(seconds=28800)))
>>> datetime.datetime.strptime('2017-12-05T15:00:24.972Z', '%Y-%m-%dT%H:%M:%S.%f%z')
datetime.datetime(2017, 12, 5, 15, 0, 24, 972000, tzinfo=datetime.timezone.utc)

GerevAI / gerev

Fix timezone parse for non-UTC timezone #96