VPython websocket not connecting on Binder

[jcoady]

VPython doesn't work on Binder due to Websocket failing to connect. Websockets to a remote computer such as binder need to connect through a proxy server such as the jupyter-server-proxy.

https://jupyter-server-proxy.readthedocs.io/en/latest/index.html

Support for jupyter-server-proxy needs to be added to VPython in order to connect a websocket through a proxy server.

[jcoady]

Fix has been checked in.