assigning local ret variable to none if the asyncio task is canceled

[rettier]

In the async namespace's trigger_event routine the ret variable is not assigned any value if the async task is canceled. This causes some IMO unnecessary exceptions.

python-socketio/socketio/asyncio_namespace.py

Lines 36 to 39 in ce44133

	try:
	ret = await handler(*args)
	except asyncio.CancelledError: # pragma: no cover
	pass

I have assigned it to none as it is done in

python-socketio/socketio/asyncio_server.py

Lines 324 to 327 in 3b702da

	try:
	ret = await self.handlers[namespace][event](*args)
	except asyncio.CancelledError: # pragma: no cover
	ret = None

[miguelgrinberg]

Thanks!