examples/webrtc/signalling: Fix compatibility with Python 3.10

- ssl module requires an explicit TLS_SERVER role
- asyncio throws a deprecation warning when using
  asyncio.get_event_loop().  Remove custom event loop handling entirely
- No need to keep the websocket server in a member variable, can use
  a future to signal exit case along with the async with context manager
  of websockets.serve()
6 jobs for !2698 with webrtc-signalling-python-3-10 in 36 seconds
merge request