DOC, BLD: sphinx 2.2.0 -> 2.3.1 to get rid of deprecation warning

Author: mattip

.circleci/config.yml

@@ -21,7 +21,7 @@ jobs:
             python3 -m venv venv
             ln -s $(which python3) venv/bin/python3.6
             . venv/bin/activate
-            pip install cython sphinx==2.2.0 matplotlib ipython
+            pip install cython sphinx==2.3.1 matplotlib ipython
             sudo apt-get update
             sudo apt-get install -y graphviz texlive-fonts-recommended texlive-latex-recommended texlive-latex-extra texlive-generic-extra latexmk texlive-xetex