Support JupyterLab 3.6

[jtpio]

To kickstart development on the extension (#1), the extension will likely target JupyterLab 4 since the extension template uses JupyterLab 4 packages by default.

However compatibility with JupyterLab 3 may still be needed for a little while. so we should check if that's indeed the case and if yes, check if the extension needs to explicitly handle that version or if it would happen to work just fine without modifications.

Using JupyterLab 4 would also allow the use of the simpler Jupyter Notebook 7 interface if needed.