Pin again sphinx version as stated in the comment. (GH-9824)

.travis.yml

@@ -54,7 +54,7 @@ matrix:
         # Sphinx is pinned so that new versions that introduce new warnings won't suddenly cause build failures.
         # (Updating the version is fine as long as no warnings are raised by doing so.)
         # The theme used by the docs is stored separately, so we need to install that as well.
-        - python -m pip install sphinx blurb python-docs-theme
+        - python -m pip install sphinx==1.8.1 blurb python-docs-theme
       script:
         - make check suspicious html SPHINXOPTS="-q -W -j4"
     - os: osx