Update sphinx from 3.1.1 to 3.1.2

Author: pyup-bot

requirements-dev.txt

@@ -7,7 +7,7 @@ docutils==0.16
 check-manifest==0.42
 readme-renderer==26.0
 pygments==2.6.1
-sphinx==3.1.1
+sphinx==3.1.2
 sphinx_rtd_theme==0.5.0
 coverage==5.1
 twine==3.1.1