Update sphinx from 1.6.6 to 1.7.0

Author: pyup-bot

requirements.txt

@@ -47,7 +47,7 @@ requests==2.18.4
 setuptools_scm>=1.15.0
 six==1.11.0
 snowballstemmer==1.2.1
-Sphinx==1.6.6
+Sphinx==1.7.0
 sphinxcontrib-websupport==1.0.1
 tox==2.9.1
 typing==3.6.2