Add syntax highlighting to ReadTheDocs

Ends up those codeblocks produced in Sphinx need a lexer in order to
to have that syntax highlighting. It seems that what it needs in order
for that to work is just iPython. So this adds iPython to the sphinx
build environment to act as a lexer.

docs/requirements.txt

@@ -4,3 +4,4 @@ sphinx-autodoc-typehints~=1.12.0
 sphinxcontrib-bibtex~=1.0.0
 nbsphinx~=0.8.6
 jinja2~=2.11.3
+ipython~=7.22.0