ci: allow ci workflow to be manually triggered

[consideRatio]

This PR enables the CI workflow to run manually by the click of a button (by repo maintainers).

Example of the button from another repo

[bollwyvl]

Sure, thanks... but it won't help debug #65, as the lab 3 excursion currently pinned rather hard against an older version of jupyter_server, and takes a bit of doing to add another factor to the matrix.

[consideRatio]

@bollwyvl ah yes I realized that recently in #65 =/

Feel free to close/merge, I think having the run workflow button is a great benefit to help catch upstream regressions but if you have everything pinned this tight in the test suite, maybe its not very relevant to re-run manually on demand.

[consideRatio]

the lab 3 excursion currently pinned rather hard against an older version of jupyter_server, and takes a bit of doing to add another factor to the matrix.

If you keep testing against py37 you could get the jupyter_server 1 automatically retained as jupyter_server 2 requires py38. Then in tests for py3.10 you would get jupyter_server 2 if jupyterlab 3.5+ is used. That way, you would have tests with jupyter_server 1 and 2 without duplicating the amounts of tests or manually handling where and when jupyter_server 2 should be adopted as that would be decided by version compatebility in requested dependencies.

[consideRatio]

I guess the crux here is that the lab3 spec includes retrolab, which pins jupyter server 1 still. See https://github.com/robots-from-jupyter/robotframework-jupyterlibrary/blob/master/.github/env_specs/lab3.yml#L13