Dynamically determine list of available and common timezones The Debian/Ubuntu packages of pytz include a patch to not install pytz's zoneinfo database and instead load data from the system zoneinfo database. This keeps pytz up to date when system timezone updates are installed which is generally a good idea.
The `pytz/__init__.py` includes `common_timezones` and `all_timezones` lists, which enumerate the time zones in the database pytz was released with. If pytz is instead using the system database, these lists may not be correct (e.g. missing newly added time zones).
Dynamically determine `all_timezones` and `common_timezones`. For `all_timezones` use `zoneinfo` from Python 3.9 and for `common_timezones` parse `/usr/share/zoneinfo/zone1970.tab`.
