Fix install and use of Jupyter Kernel for docs

[IvanIsCoding]

Tries to fix the same problem as #844 but without pinning

For some reason the python3 kernel stopped being installed togethe with Jupyter, hence:

• we install the kernel on the tox environment
• we explicity use the kernel in the Sphinx configuration

[coveralls]

Pull Request Test Coverage Report for Build 4496904050

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 97.114%

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Files with Coverage Reduction	New Missed Lines	%
src/shortest_path/all_pairs_bellman_ford.rs	1	98.88%

Totals
Change from base Build 4475309435:	0.02%
Covered Lines:	13896
Relevant Lines:	14309

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💛 - Coveralls

[mtreinish]

Thanks for fixing this for real, my eager pinning wasn't sufficient. I'm a bit surprised that we need to manually install ipykernel I'm guessing it was getting pulled in transitively by something before and a package was missing it in its requirements list. But since this is passing CI it should be good to go.

[mtreinish]

I'm quite surprised this isn't getting pulled in anymore. I would expect jupyter-sphinx to end up pulling this in.