Implement abstract method can.Listener.stop() in MessageListener. (canopen-python#422)

(cherry picked from commit a973919)

Author: acolomb

canopen/network.py

@@ -402,6 +402,9 @@ def on_message_received(self, msg):
             # Exceptions in any callbaks should not affect CAN processing
             logger.error(str(e))
 
+    def stop(self) -> None:
+        """Override abstract base method to release any resources."""
+
 
 class NodeScanner:
     """Observes which nodes are present on the bus.