fix datetime without TZ warnings

here is a sample warning:

RuntimeWarning: DateTimeField Holiday.datetime_start received a naive datetime (2020-11-13 12:19:24.909491) while time zone support is active

added refactor label

created merge request !272 (merged) to address this issue

mentioned in merge request !272 (merged)

changed title from fix datetime without TX warnings to fix datetime without TZ warnings

closed with merge request !272 (merged)

mentioned in commit 9bba8715