[roottest] Use nbconvert as a library in nbdiff.py

[guitargeek]

Using nbconvert as a Python library is less fragile, because otherwise there is ambiguity how the command line util should be called (the name changed over the versions as it migrated from ipython to jupyter).

This will make it possible to run the notebook tests also on the Fedora Rawhide build that is linked against the Python debug version, and hopefully also fix the test failures on the macOS platforms when a Python interpreter other than the system Python is used.

[github-actions]

Test Results

    21 files      21 suites   3d 15h 15m 36s ⏱️
 3 776 tests  3 431 ✅   0 💤 345 ❌
77 432 runs  76 699 ✅ 388 💤 345 ❌

For more details on these failures, see this check.

Results for commit 388d1fd.

♻️ This comment has been updated with latest results.

[pcanal]

LFTM (but see the set of questions)

[guitargeek]

Thanks for the review! I have added one commit that addresses your comments. In particular, the reference notebooks are not updated to Python 3, so we don't have to manually set the kernel name in the nbdiff.py script.

[guitargeek]

The failures in Fedora Rawhide are unrelated: they come from an update of the system gtest, and because the builds are incremental it still tries to link against the old version.

Maybe it's possible to also "hash" the docker image somehow, and when it changed do a clean build. To be seen later if this should continue to annoy us.