Drop Travis/Appveyor

[bollwyvl]

The delays on travis are getting me down on #5907. And appveyor is... appveyor.

It's probably a relatively quick PR to get GitHub Actions spun up and get results faster, and increase platform coverage. Anybody violently against?

Update: taking a look here: bollwyvl#1

[bollwyvl]

In exploring this, terminals fail on win/39:

andfoy/pywinpty#134

[bollwyvl]

Have all of the python tests running on 3.6-3.9 + pypy:

https://github.com/bollwyvl/notebook/actions/runs/417927458

• some skips for pypy
• ignore terminals on py39

[minrk]

We've been busy migrating ~everything on JupyterHub to GHA and been super happy with it. Migrating from Travis hasn't been too tough in most cases. Feel free to open that PR!

[Zsailer]

Done in #5938.

Looks like terminals on Windows and Python 3.9 are still an issue, but the rest of the tests appear to be running okay 👍