gh-132413: Fix crash in _datetime when used at shutdown

[neonene]

This converts the IsoCalendarDate heap type back to the static type used in 3.12,
so that the _datetime will not heavily depend on its module state.

Unlike #132665, this does not extend the interpreter state.

Previous attempt

Currently, the _datetime module is cached in sys.modules. We acquire the cache through a weakref in each interpreter's dict, checking if a module was once loaded before in order not to re-initialize the C-API static types.
This PR directly caches the module into the interp dict, allowing the cache to be released only when replaced with a new one. It may be memory inefficient, but the module needs to keep alive even after sys.modules is unavailable.

• Issue: SIGSEV in datetime.timedelta (possibly from datetime's C delta_new) #132413

[neonene]

cc @Jacoblightning@ericsnowcurrently

[neonene]

Please take a look.