Pinned IPython version to fix bug in documentation build (#747)

Author: JonathanWenger

docs/notebook-requirements.txt

@@ -1,4 +1,8 @@
 # Jupyter notebooks
+ipython<=8.6.0
+# ipython 8.7 introduced a bug regarding pygments lexer support
+# https://github.com/spatialaudio/nbsphinx/issues/24
+# https://github.com/ipython/ipython/issues/13845
 jupyter
 
 # Notebook conversion