API: Default to stdlib timezone objects instead of pytz
i.e. when we get `pd.Timestamp.now("UTC")` we should return a Timestamp with `datetime.timezone.utc` rather than `pytz.UTC`.  Similarly when we parse an ISO8601 datetime we should use tzinfo of `timezone(timedelta(seconds=val*60))` instead of `pytz.FixedOffset(val)`

This isn't _that_ hard to implement, but doing it breaks a couple dozen tests where we are currently checking for pytz objects.  This would technically be an API change, so putting it up for discussion before implementing it.