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/_posixsubprocess.c

@@ -1140,6 +1140,7 @@ static PyMethodDef module_methods[] = {
 };
 
 static PyModuleDef_Slot _posixsubprocess_slots[] = {
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };