use macros

Objects/stringobject.c

@@ -3948,7 +3948,7 @@ static PyObject *
 string_sizeof(PyStringObject *v)
 {
 	Py_ssize_t res;
-	res = PyStringObject_SIZE + v->ob_size * v->ob_type->tp_itemsize;
+	res = PyStringObject_SIZE + PyString_GET_SIZE(v) * Py_TYPE(v)->tp_itemsize;
 	return PyInt_FromSsize_t(res);
 }