Bump sphinx from 2.1.1 to 2.1.2 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.1.1 to 2.1.2.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v2.1.1...v2.1.2)

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

docs/requirements.txt

@@ -1,4 +1,4 @@
 lesscpy==0.13.0
-Sphinx==2.1.1
+Sphinx==2.1.2
 sphinx-autobuild==0.7.1
 sphinx-rtd-theme==0.4.3