Fix python syntax highlighting in remote readthedocs build, in notebooks

[drbenvincent]

We used to get appropriate syntax highlighting of Python code in both local and remote builds of the docs. And that went for .rst, .md, and .ipynb files.

Currently I still get good syntax highlighting locally.

But the problem is that syntax highlighting has broken just for ipynb files and just in the remote readthedocs build.

Example of correctness in local docs build

Example of incorrect syntax highlighting in remote readthedocs build

Warning

The relevant warning in the logs of the remote readthedocs build is:

WARNING: Pygments lexer name 'ipython3' is not known

[drbenvincent]

Note, adding ["IPython.sphinxext.ipython_console_highlighting", "IPython.sphinxext.ipython_directive"] to the extensions list in conf.py is not an acceptable solution. It gives the ugly lime green fill colour of the code cells

[drbenvincent]

It looks like this might be an ipython issue scikit-hep/pyhf#2067

[drbenvincent]

This did fix the problem. Fingers crossed it will be resolved in 8.7.1. If not, this might reappear, in which case this problem will reemerge.