ARROW-5487: [Docs] Fix Sphinx failure
ARROW-5487: [Docs] Fix Sphinx failure

Regression reported upstream at https://github.com/sphinx-doc/sphinx/issues/6436, but easily fixable on our side.

Author: Antoine Pitrou <antoine@python.org>

Closes #4455 from pitrou/ARROW-5487-sphinx-ci-failure and squashes the following commits:

fe428179b <Antoine Pitrou> ARROW-5487:  Fix Sphinx failure

laurentgo/arrow,renesugar/arrow,kou/arrow,icexelloss/arrow,wesm/arrow,wesm/arrow,laurentgo/arrow,renesugar/arrow,wesm/arrow,laurentgo/arrow,laurentgo/arrow,apache/arrow,renesugar/arrow,majetideepak/arrow,cpcloud/arrow,cpcloud/arrow,apache/arrow,kou/arrow,majetideepak/arrow,apache/arrow,laurentgo/arrow,kou/arrow,renesugar/arrow,wesm/arrow,icexelloss/arrow,icexelloss/arrow,apache/arrow,wesm/arrow,cpcloud/arrow,xhochy/arrow,majetideepak/arrow,wesm/arrow,apache/arrow,icexelloss/arrow,icexelloss/arrow,kou/arrow,icexelloss/arrow,renesugar/arrow,wesm/arrow,apache/arrow,laurentgo/arrow,xhochy/arrow,kou/arrow,cpcloud/arrow,cpcloud/arrow,majetideepak/arrow,laurentgo/arrow,majetideepak/arrow,cpcloud/arrow,laurentgo/arrow,laurentgo/arrow,cpcloud/arrow,laurentgo/arrow,laurentgo/arrow,icexelloss/arrow,icexelloss/arrow,icexelloss/arrow,kou/arrow,renesugar/arrow,apache/arrow,majetideepak/arrow,majetideepak/arrow,majetideepak/arrow,icexelloss/arrow,cpcloud/arrow,kou/arrow,xhochy/arrow,xhochy/arrow,kou/arrow,kou/arrow,kou/arrow,renesugar/arrow,apache/arrow,renesugar/arrow,renesugar/arrow,laurentgo/arrow,kou/arrow,renesugar/arrow,majetideepak/arrow,cpcloud/arrow,xhochy/arrow,xhochy/arrow,renesugar/arrow,laurentgo/arrow,majetideepak/arrow,majetideepak/arrow,xhochy/arrow,renesugar/arrow,cpcloud/arrow,wesm/arrow,renesugar/arrow,kou/arrow,icexelloss/arrow,xhochy/arrow,icexelloss/arrow,kou/arrow,cpcloud/arrow,xhochy/arrow,wesm/arrow,laurentgo/arrow,cpcloud/arrow,apache/arrow,icexelloss/arrow,xhochy/arrow,apache/arrow,wesm/arrow,icexelloss/arrow,majetideepak/arrow,xhochy/arrow,xhochy/arrow,majetideepak/arrow,renesugar/arrow,kou/arrow,majetideepak/arrow,apache/arrow,wesm/arrow,laurentgo/arrow,cpcloud/arrow,xhochy/arrow,xhochy/arrow,apache/arrow,majetideepak/arrow,apache/arrow,xhochy/arrow,apache/arrow,renesugar/arrow,wesm/arrow,wesm/arrow,wesm/arrow,cpcloud/arrow