Add GitHub workflow to automatically update to newer JupyterLab versions

[jtpio]

To ease maintenance, it would be useful to have a script that checks what the latest version of JupyterLab is, and automatically opens a PR that bumps to the newer packages.

Some ideas and possible config options for that script:

• run as a cron, and / or via a workflow dispatch on demand
• support updating to a specific version, latest (stable) or next (pre-release)
• this might also require being able to specify a branch, as multiple versions of notebook (stable and pre-release) can be developed at the same time

JupyterLab Desktop has something similar: jupyterlab/jupyterlab-desktop#311

[jtpio]

In some cases there is more work to do than just updating the versions (for example integrating some upstream plugins more natively in Notebook). But at least this script would help with the repetitive parts.