This behaviour is deprecated and will be removed in future versions
of Python. Technically, it could be considered a bug (invalid usage
causing different behaviour from the expected one), and in practice
it should not break much code (because .get_event_loop() would likely
be the same event loop anyway).
This prevents us from locking forever on any task that doesn't
rely on cancellation tokens, in this case, Connection.recv()'s
_recv_queue.get() would never complete after the server closed
the connection.
Additionally, working with cancellation tokens in asyncio is
somewhat annoying to do.
Last but not least removing the Connection._disconnected future
avoids the need to use its state (if an exception was set it
should be retrieved) to prevent asyncio from complaining, which
it was before.
