Support use in notebook v7

[consideRatio]

This project has been providing functionality for both jupyterlab and the classic notebook interface. As part of upgrading to support jupyterlab 3+, we've stopped working with classic notebook interface entirely - no matter if it was based on v6 or v7 of notebook.

I think its in scope to support notebook v7 for this project, but that requires maintenance - I'm updating the README file to reflect that via #24 and opening this issue to be a reference on that work.

[consideRatio]

Here is a notebook v7 extension we could mimic how to set things up I think, but I've not found docs on what, if anything, is required to extend the notebook v7 interface as compared to a normal jupyterlab extensions.

Do we need _jupyter_nbextension_paths for example?