Bump sphinx from 1.8.3 to 1.8.4

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 1.8.3 to 1.8.4.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v1.8.3...v1.8.4)

Signed-off-by: dependabot[bot] <support@dependabot.com>

requirements.txt

@@ -11,5 +11,5 @@ gunicorn==19.9.0
 pycodestyle==2.5.0
 pydocstyle==3.0.0
 requests==2.21.0
-Sphinx==1.8.3
+Sphinx==1.8.4
 Werkzeug==0.14.1