jupyter-notebook 7 is now in experimental, and emperor shows
autopkgtest failures with this version.

The proximate issue is trying to install a nbextension by importing
a method which no longer exists:

=================================== FAILURES ===================================

This bug was initially about autopkgtest. Then I tagged it "ftbfs"
because it was also a build problem.

But now the package builds ok again, and based on the green pipeline
in salsa, it looks like autopkgtest is working again.

So I hereby declare this fixed in the latest version.

(If I'm mistaken, please reopen).

[ Note: This package had my attention because it ftbfs with nocheck
  build profile, but I'm going to let it propagate to testing before
  reporting that ].

Thanks.