test: py3: port test_subprotocol_negotiation.py
test: py3: port test_subprotocol_negotiation.py

Another abuse of the websockets package here, this time to control the
exact format of the Sec-WebSocket-Protocol header while still allowing
the internal machinery to correctly select a subprotocol.

jchampio/apache-websocket,jchampio/apache-websocket,jchampio/apache-websocket