gh-116322: Add Py_mod_gil module slot (#116882)

This PR adds the ability to enable the GIL if it was disabled at
interpreter startup, and modifies the multi-phase module initialization
path to enable the GIL when loading a module, unless that module's spec
includes a slot indicating it can run safely without the GIL.

PEP 703 called the constant for the slot `Py_mod_gil_not_used`; I went
with `Py_MOD_GIL_NOT_USED` for consistency with gh-104148.

A warning will be issued up to once per interpreter for the first
GIL-using module that is loaded. If `-v` is given, a shorter message
will be printed to stderr every time a GIL-using module is loaded
(including the first one that issues a warning).

Modules/_datetimemodule.c

@@ -6984,6 +6984,9 @@ PyInit__datetime(void)
     PyObject *mod = PyModule_Create(&datetimemodule);
     if (mod == NULL)
         return NULL;
+#ifdef Py_GIL_DISABLED
+    PyModule_ExperimentalSetGIL(mod, Py_MOD_GIL_NOT_USED);
+#endif
 
     if (_datetime_exec(mod) < 0) {
         Py_DECREF(mod);