Drop `python 3.6` and `traitlets 4` from CI test

[trungleduc]

Voila depends heavily on nbconvert and nbconvert required minimum python 3.7 and traitlets 5 since version 6.1.0. Should we drop the configuration for testing python 3.6 and add the test for python 3.10?

[trungleduc]

@jtpio should we go for it?

[jtpio]

I would say yes, especially since nbconvert now requires 3.7+.

Not sure whether it should go in a 0.3.1 just yet though. We might want to wait for the final 3.6 EOL before releasing.

[jtpio]

Maybe we could start testing on 3.7 and 3.10 on CI as a first step. I'll open a PR.

[trungleduc]

The latest release of jupyter_server is not compatible with traitlets 4 jupyter-server/jupyter_server#661

[martinRenou]

Remove traitlets 4 test in #1060