Extension error in sphinx website build

[luukvdmeer]

Sphinx build (i.e. the documentation website) now fails with the following error:

Extension error:
Could not import extension nbsphinx (exception: lxml.html.clean module is now a separate project lxml_html_clean.
Install lxml[html_clean] or lxml_html_clean directly.)
Error: Process completed with exit code 2.

I have not had time to look into it but feel like this is a bug in nbsphinx and not semantique. More research needed