[backen] catch RuntimeError: Set changed size during iteration

[manfred-ackermann]

Mär 13 18:06:23 doorpi python[22897]: INFO:root:remaining time to open: 12 seconds
Mär 13 14:40:26 doorpi python[22897]: File "/usr/local/doorpi-agent/venv/local/lib/python2.7/site-packages/gpiozero/mixins.py", line 301, in _fire_act
Mär 13 14:40:26 doorpi python[22897]: self.when_activated()
Mär 13 14:40:26 doorpi python[22897]: File "doorpi.py", line 447, in handle_ring
Mär 13 14:40:26 doorpi python[22897]: DoorSocketHandler.send_update(tornado.escape.json_encode(payload))
Mär 13 14:40:26 doorpi python[22897]: File "doorpi.py", line 494, in send_update
Mär 13 14:40:26 doorpi python[22897]: for waiter in cls.waiters:
Mär 13 14:40:26 doorpi python[22897]: RuntimeError: Set changed size during iteration
Mär 13 14:40:26 doorpi python[22897]: INFO:root:handling RING

Nothing grave but ugly in the log.