webrtc_sendrecv.py: Fix deprecation warning with Python 3.10

webrtc_sendrecv.py: Fix deprecation warning with Python 3.10

asyncio.get_event_loop() will not implicitly create a new event loop
in a future version of Python, so we need to do that explicitly.

```
webrtc_sendrecv.py:188: DeprecationWarning: There is no current event loop
  loop = asyncio.get_event_loop()
```

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1821>