update for Onyx availability

[kcpevey]

I'm having trouble authenticating, so I can't test this fully, but at a minimum the pyit_demo.ipynb verbage needs to be updated to include Onyx as a system option.

[sdc50]

I was able to authenticate on Onyx. I'll update the demo to include Onyx.

[kcpevey]

Leaving this open as a reminder to document.