Update sphinx from 1.8.1 to 2.0.1

Author: pyup-bot

requirements_dev.txt

@@ -5,7 +5,7 @@ watchdog==0.9.0
 flake8==3.6.0
 tox==3.5.3
 coverage==4.5.1
-Sphinx==1.8.1
+Sphinx==2.0.1
 twine==1.12.1
 sphinx_rtd_theme==0.4.2
 pytest==3.9.3