Fix typing compatibility with Python 3.9 (EOL, btw)

Author: jorenham

zmq/sugar/frame.py

@@ -3,6 +3,8 @@
 # Copyright (C) PyZMQ Developers
 # Distributed under the terms of the Modified BSD License.
 
+from __future__ import annotations
+
 from typing import Literal, overload
 
 import zmq