python 3.9 added the zoneinfo module to the standard library.
It can be used like we use pytz.
Add support to use zoneinfo if present so user can specify symbolic
timezones without having to pip install pytz.

See also: https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html

There are some subtleties in differences between from dateutil.tz and pytz
regarding when values are calculated and differences in edge case calculations
(e.g. repeated times due to daylight saving changes).