nmt: fix update_heartbeat to use bytes

Author: bizfsc

canopen/nmt.py

@@ -270,7 +270,7 @@ def stop_heartbeat(self):
 
     def update_heartbeat(self):
         if self._send_task is not None:
-            self._send_task.update([self._state])
+            self._send_task.update(bytes([self._state]))
 
 
 class NmtError(Exception):