Update sphinx from 1.7.9 to 1.8.0

Author: pyup-bot

requirements_dev.txt

@@ -23,7 +23,7 @@ pytest-mock==1.10.0
 pytest-cov==2.6.0
 
 # docs
-Sphinx==1.7.9
+Sphinx==1.8.0
 Pygments==2.2.0
 sphinxcontrib-asyncio==0.2.0
 sphinx-rtd-theme==0.4.1