Remove dependency on pytz

[dairiki]

Since the introduction of PEP 614 PEP 615 (the zoneinfo module in the standard library, available since python 3.9), the future, apparently, is to transition away from pytz to zoneinfo for timezone support.

Babel, since 2.12, will now work with either pytz or zoneinfo, and no longer requires pytz on pythons >= 3.9.

This change also fixes some hokey-ness in the previous implementation of DateTimeType.

Related Issues / Links

See #1109.

Description of Changes

• Added unit test(s) covering the changes (if testable)

[dairiki]

Hmmph. Apparently

Some systems, including notably Windows systems, do not have an IANA database available, and so for projects targeting cross-platform compatibility that require time zone data, it is recommended to declare a dependency on tzdata.

I've added pytz back to our dependency list, but only on Windows. Perhaps, for uniformity, we need to keep it for all platforms? OTOH, it's not the end of the world if a user has to manually install pytz if their system doesn't provide a suitable timezone database and they want to use named timezones in their datetime fields.

[CAM-Gerlach]

Hmm; is there a reason to not just add a dependency on tzdata instead, as the message indicates? That just ships the time zone db that zoneinfo needs rather than all of pytz.

[dairiki]

Hmm; is there a reason to not just add a dependency on tzdata instead, as the message indicates?

Because I can't read. Thank you.