bpo-41568: Fix refleaks in zoneinfo subclasses (GH-21907)

bpo-41568: Fix refleaks in zoneinfo subclasses (GH-21907)

* Fix refleak in C module __init_subclass__

This was leaking a reference to the weak cache dictionary for every
ZoneInfo subclass created.

* Fix refleak in ZoneInfo subclass's clear_cache

The previous version of the code accidentally cleared the global
ZONEINFO_STRONG_CACHE variable (and inducing `ZoneInfo` to create a new
strong cache) on calls to a subclass's `clear_cache()`. This would not
affect guaranteed behavior, but it's still not the right thing to do
(and it caused reference leaks).