gh-99113: Add Py_MOD_PER_INTERPRETER_GIL_SUPPORTED (gh-104205)

Here we are doing no more than adding the value for Py_mod_multiple_interpreters and using it for stdlib modules.  We will start checking for it in gh-104206 (once PyInterpreterState.ceval.own_gil is added in gh-104204).

Modules/_elementtree.c

@@ -4419,6 +4419,9 @@ error:
 
 static struct PyModuleDef_Slot elementtree_slots[] = {
     {Py_mod_exec, module_exec},
+    // XXX gh-103092: fix isolation.
+    {Py_mod_multiple_interpreters, Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED},
+    //{Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };