Switch to pytest and jupyter-server

[fcollonval]

For some unknown reasons, switching to the jupyter-server pytest logic is not working for the API tests on the CI - it works locally for me.

This needs investigation but should not delay further the update for JupyterLab 3.

[github-actions]

👈 Launch a binder notebook on the branch mamba-org/gator/ft/switch-pytest

[fcollonval]

Thanks @jtpio for jumping in - I definitely did something wrong when extracting it from #111
But there is a deeper trouble with coroutine tests - see CI before I extract this https://github.com/mamba-org/gator/runs/1845810262?check_suite_focus=true

My current goal is to finish as quickly as possible #111 - the only thing left is the standalone demo in binder. So that we can release the JupyterLab 3 version. Then I'll rebase this PR.