[issue2551254] add support for zoneinfo (python 3.9+) in addition to pytz

New submission from John Rouillard:

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.

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components: Web interface
messages: 7706
nosy: rouilj
severity: minor
status: new
title: add support for zoneinfo (python 3.9+) in addition to pytz
type: rfe

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