Feature or enhancement

There are a few pieces to this:

1. Add a Py_mod_gil slot, as described in PEP 703, that multi-phase init modules can use to indicate that they support free-threading.
2. Add a PyModule_ExperimentalSetGIL() function (discussed here) that single-phase init modules can use in place of Py_mod_gil.
3. Mark all built-in modules as free-threading compatible with one of the above mechanisms.
4. Enable the GIL while loading a C module and leave it permanently enabled if the module does not declare free-threading compatibility.

1-3 are addressed in gh-116882; 4 will be addressed in a separate PR.

Linked PRs

• gh-116322: Add Py_mod_gil module slot #116882
• gh-116322: Enable the GIL while loading C extension modules #118560
• gh-116322: Rename PyModule_ExperimentalSetGIL to PyUnstable_Module_SetGIL #118645
• gh-116322: Fix typo in the #ifdef check #122268
• gh-116322: Fix typo in moduleobject.c #122282
• [3.13] gh-116322: Fix typo in the GH-ifdef check (GH-122268) #122284