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…

spack.yaml

@@ -6,6 +6,8 @@ spack:
     - py-ipykernel
     - py-notebook
     - py-ipycanvas
+    # "collab"-specific constraint to match ("jupyterlab_widgets") in the base image
+    - py-ipywidgets@:7.7
     # Collab utils
     - clb-nb-utils@0.1.0
     # EBRAINS tools