Merge pull request mohankishore#331 from mohankishore/pyup-update-tox-3.5.3-to-3.9.0

Update tox to 3.9.0

Author: mohankishore

requirements_dev.txt

@@ -3,7 +3,7 @@ bumpversion==0.5.3
 wheel==0.33.4
 watchdog==0.9.0
 flake8==3.6.0
-tox==3.5.3
+tox==3.9.0
 coverage==4.5.1
 Sphinx==1.8.1
 twine==1.12.1