Update sphinx from 2.0.1 to 2.1.0

Author: pyup-bot

requirements_dev.txt

@@ -5,7 +5,7 @@ watchdog==0.9.0
 flake8==3.7.7
 tox==3.12.1
 coverage==4.5.3
-Sphinx==2.0.1
+Sphinx==2.1.0
 twine==1.13.0
 
 pytest==4.5.0