gh-104108: Add the Py_mod_multiple_interpreters Module Def Slot (gh-104148)

I'll be adding a value to indicate support for per-interpreter GIL in gh-99114.

Modules/_testmultiphase.c

@@ -884,3 +884,22 @@ PyInit__test_module_state_shared(void)
     }
     return module;
 }
+
+
+/* multiple interpreters supports */
+
+static PyModuleDef_Slot non_isolated_slots[] = {
+    {Py_mod_exec, execfunc},
+    {Py_mod_multiple_interpreters, Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED},
+    {0, NULL},
+};
+
+static PyModuleDef non_isolated_def = TEST_MODULE_DEF("_test_non_isolated",
+                                                      non_isolated_slots,
+                                                      testexport_methods);
+
+PyMODINIT_FUNC
+PyInit__test_non_isolated(void)
+{
+    return PyModuleDef_Init(&non_isolated_def);
+}