Bump sphinx from 1.7.4 to 2.1.2

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 1.7.4 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@v1.7.4...v2.1.2)

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

docs/requirements.txt

@@ -1,7 +1,7 @@
 # This file is used to setup env
 # to generate documentation.
 
-sphinx==1.7.4
+sphinx==2.1.2
 sphinx-readable-theme==1.3.0
 sphinxcontrib-napoleon==0.6.1
 recommonmark==0.4.0