Use bocadillo and async+await to get new pty output

[cs01]

pyxterm.js uses flask+flask-socketio on the backend. To get new output from the pty, it runs socketio.sleep() and checks for new output in an endless loop.

It would be better if it could use async+await to immediately forward the new output over a websocket. The new async web framework Bocadillo would be a good alternative to try since it now has websocket support bocadilloproject/bocadillo#75.

https://github.com/bocadilloproject/bocadillo