Add constraint for older py-ipywidgets

In the collab base image there's jupyterlab-widgets@1.0.2 (installed via pip), which seems incompatible with ipywidgets@8.0.2 (in the current experimental deployment) → let's add a version constraint for ipywidgets to stay in the 7.7 range (7.7.0 worked before, current stable).

This is a site-specific change that should go into the site/collab thingy…