gh-71587: Drop local reference cache to `_strptime` module in `_datetime` (gh-120224)

gh-71587: Drop local reference cache to `_strptime` module in `_datetime` (gh-120224)

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()).