use the 's' format code instead of 'U'

Parser/asdl_c.py

@@ -722,7 +722,7 @@ static PyTypeObject* make_type(char *type, PyTypeObject* base, char**fields, int
         }
         PyTuple_SET_ITEM(fnames, i, field);
     }
-    result = PyObject_CallFunction((PyObject*)&PyType_Type, "U(O){sOss}",
+    result = PyObject_CallFunction((PyObject*)&PyType_Type, "s(O){sOss}",
                     type, base, "_fields", fnames, "__module__", "_ast");
     Py_DECREF(fnames);
     return (PyTypeObject*)result;