[Python] Drop pytz for timezone support (default to use datetime.timezone / zoneinfo)

We already made pytz an optional dependency a while ago (ARROW-15580, https://github.com/apache/arrow/pull/12522), so you can now convert arrow timestamp with tz to python without having pytz. In the case pytz is not installed, we "fall back" to `datetime.timezone(datetime.timedelta(..))` for fixed offsets and `zoneinfo.ZoneInfo(..)` for known time zones.

However, we should make this fall back the default at some point (and potentially dropping the option to convert to pytz automatically altogether).
Pandas starts to do this in pandas 2.0 for fixed offsets (https://pandas.pydata.org/docs/dev/whatsnew/v2.0.0.html#utc-and-fixed-offset-timezones-default-to-standard-library-tzinfo-objects)

One complication for this is that the `zoneinfo` module is only available in the standard library starting with Python 3.9, while we still support older versions (which would require https://pypi.org/project/backports.zoneinfo/ if we want consistent behaviour across all python versions)

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