Apply PR eclipse-paho#884

Author: JamesParrott

src/paho/mqtt/client.py

@@ -4213,7 +4213,7 @@ def _handle_pubrel(self) -> MQTTErrorCode:
                 # prevents multiple callbacks for the same message.
                 message = self._in_messages.pop(mid)
                 self._handle_on_message(message)
-                self._inflight_messages -= 1
+                # self._inflight_messages -= 1
                 if self._max_inflight_messages > 0:
                     with self._out_message_mutex:
                         rc = self._update_inflight()