Allow C extensions to declare compatibility with free-threading

[swtaarrs]

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

[swtaarrs]

What warning category should this issue? ImportWarning sounds like a good fit at first, but it's ignored by the default warning filters, which we don't want. Of the categories not ignored by default, RuntimeWarning seems like the best fit to me.