gh-144330: Initialize staticmethod/classmethod callables in tp_new

[Aniketsy]

#144364

• Issue: classmethod and staticmethod can crash with NULL / uninitialized callables #144330

[Aniketsy]

@vstinner Apologies for the previous PR. I’ve addressed the review suggestions in this one. Thanks!

[vstinner]

With this change, cm->cm_callable cannot be NULL anymore:

• cm_repr(): So you can remove cm->cm_callable != NULL (revert your change).
• cm_dealloc(): Py_XDECREF(cm->cm_callable); can be replaced with Py_DECREF(cm->cm_callable);.
• cm_descr_get(): I suggest to replace if (cm->cm_callable == NULL) with if (cm->cm_callable == Py_None).
• cm_init(): replace Py_XSETREF(cm->cm_callable, ...) with Py_SETREF(cm->cm_callable, ...).

[vstinner]

I wrote a different approach: parse arguments in cm_new()/sm_new(): PR gh-144469.

[vstinner]

I merged #144469 instead. Thanks for your contribution anyway!

[vstinner]

I wrote a simpler change for 3.13 and 3.14: #144498 I added you as a co-author since I copied some code from your PR.

[Aniketsy]

Thanks