[3.12] gh-71587: Drop local reference cache to _strptime module in _datetime (GH-120224)

[neonene]

The _strptime module object was cached in a static local variable (in the datetime.strptime() implementation). That's a problem when it crosses isolation boundaries, such as reinitializing the runtme or between interpreters. This change fixes the problem by dropping the static variable, instead always relying on the normal sys.modules cache (via PyImport_Import()).

(cherry picked from commit 127c1d2, AKA gh-120224)

• Issue: Datetime NoneType after calling Py_Finalize and Py_Initialize #71587

[neonene]

Test not included, as the test_embed case requires the following changes:

1. [3.12]: gh-113055: Use pointer for interp->obmalloc state #118618

2. The rollback to the reference count management for interned strings
   Interned strings are immortal, despite what the documentation says #113993

[neonene]

This now avoids ABI changes. I'm not sure #118618 (comment) can be applied here.

I'm fine with this.