[Debian-science-sagemath] Jupyter notebook dependency

[Gordon Ball]

Sagemath currently builds the package sagemath-jupyter, which in turn
depends on python-notebook (ie, the python 2 notebook server).

Jupyter notebook 6.0 has recently been released, which drops support for
python 2.7.

Looking in the source for sagemath, the only place I can find where the
notebook is directly imported from is in
src/sage/repl/ipython_kernel/install.py:313, then only to test whether
the notebook server is installed. In the debian package, this file looks
superfluous to me. This suggests it should be possible to depend on the
python3 notebook server without compromising the ability to run the
custom sagemath python2 kernel. (It's possible my fairly simple search
here has missed some deeper entanglement).

Would it be possible to change the notebook dependency so that I can
update jupyter-notebook without causing uninstallable?

Thanks

Gordon