Broken notebooks using ipywidget -> nglview (`AttributeError`)

[dominiquesydow]

Our weekly CI failed. All notebooks using ipywidgets are broken with new version 8; affects e.g. nglview.

Error message:
AttributeError: 'super' object has no attribute '_ipython_display_'

See discussions here:
nglviewer/nglview#1032
jupyter-widgets/ipywidgets#3552

Solution for now:
Pin ipywidgets to version 7, see nglviewer/nglview#1033.

https://github.com/volkamerlab/teachopencadd/blob/daeb016bb89feb93813be925085aff47885225ce/devtools/test_env.yml#L11