Minor changes to make the code compatible with django 4.0
Note that we can't actually upgrade to django 4.0 because that requires
python 3.8, and www.eleves only has python 3.7, so we need to upgrade
www.eleves first.
This implements a structured user group model, propagating its changes
to vanilla Django groups. It features cycle detection at save-time,
although this cycle detection can only be seen as a fallback and forms
should check the acyclicity of their changes before committing using the
provided class method.
