websocket message handler cleanup breaks with websockets 8.0

[edaniszewski]

When doing some local dev work and updating deps, websockets got bumped to 8.0, which caused some test failures around:

>       if ws.close_connection_task is not None:
E       AttributeError: 'WebSocketCommonProtocol' object has no attribute 'close_connection_task'

we currently use the close_connection_task to add a callback to clean up the message handler tasks once the connection is terminated.

Pinning the websockets version to 7.0 works for now. This issue is to investigate how to update the message handler task cleanup such that it works with the latest version of websockets.