gh-117398: datetime: Support per-interpreter C-API

[neonene]

This places C-API structure on a module state, which can be accessed through PyInterpreterState or a capsule.

Doc:

For modules using multi-phase initialization, e.g. PyModule_FromDefAndSpec(),
a separate module object is created and initialized for each interpreter.

• Issue: Isolate the _datetime extension module #117398

[Eclips4]

Can you explain why do you pass a NULL as a capsule destructor?
Is there any reason not to have destructor for this capsule?

[neonene]

In this case, the capsule just views a memory allocated and freed by the moduleobject.

cpython/Objects/moduleobject.c

Lines 739 to 740 in bfc57d4

	if (m->md_state!=NULL)
	PyMem_Free(m->md_state);

[neonene]

No futher experiment here.

[neonene]

On second thought, exposing a module state to a user is dangerous.