Add a feature to optionally build Jupyter Notebook off of JupyterLab PRs

[afshin]

Since many PRs in JupyterLab core packages are upstream of Jupyter Notebook v7+, it would be helpful if an issue that specifically manifests in Jupyter Notebook or is easier to diagnose, test, and resolve in Notebook than in JupyterLab can include a build of Jupyter Notebook that uses the code contained within a JupyterLab PR.

Problem

In particular, when working on the accessibility of Jupyter Notebook, which is farther along than JupyterLab in improvement of accessibility, it often requires PRs against core JupyterLab packages.

Proposed Solution

To test these accessibility fixes in Jupyter Notebook, it would be helpful if we could have the CI that generates testable environments in Binder also include Jupyter Notebook, optionally.

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CC: @trallard @tonyfast @gabalafou

[tonyfast]

thanks for opening this issue @afshin. it seems we'd really need this feature in the binder build. having this feature will make screen reader testing a lot quicker for testers. it seems we are running in situations where accessibility improvements in the notebook are achieved with changes in either jupyterlab or lumino.

can we use some of the pkg tags to trigger builds? like when pkg:notebook or pkg:filebrowser is tagged then we build a version of retro/classic. in theory, any packages that notebook depends on, we'd want to consider a build to test. an alternative would be to introduce a binder:notebook.