set(all_timezones_set) returns empty unless called after all_timezones_set

[Deep Chowdhury]

The set(all_timezones_set) return empty unless we call just all_timezones_set once.
Once we call all_timezones_set, the set(all_timezones_set) also gets populated and works fine.

>>> import pytz
>>> set(pytz.all_timezones_set)
set()
>>> 'Asia/Dubai' in set(pytz.all_timezones_set)
False
>>> 'Asia/Dubai' in pytz.all_timezones_set
True
>>> 'Asia/Dubai' in set(pytz.all_timezones_set)
True

pytz version: 2020.1
python version: 3.7.3

[Stuart Bishop (stub)]

I think this is https://bugs.launchpad.net/pytz/+bug/1435617 again, which I don't think can be fixed for older Python versions. It may be possible to drop lazy.py with newer Python and an alternative lazy loading mechanism.