Avoid deprecation warning for setDaemon (#364)

Author: ice1x

nebula3/gclient/net/ConnectionPool.py

@@ -338,5 +338,5 @@ def _period_detect(self):
         self.update_servers_status()
         self._remove_idle_unusable_connection()
         timer = Timer(self._configs.interval_check, self._period_detect)
-        timer.setDaemon(True)
+        timer.daemon = True
         timer.start()