Merge pull request #362 from fantix/pyup-update-pytest-3.8.2-to-3.9.1

Update pytest to 3.9.1

Author: fantix

requirements_dev.txt

@@ -15,7 +15,7 @@ quart==0.6.7;python_version>="3.6"
 coverage==4.5.1
 flake8==3.5.0
 tox==3.5.2
-pytest==3.8.2
+pytest==3.9.1
 pytest-runner==4.2
 pytest-asyncio==0.9.0
 pytest-tornado==0.5.0