Update pytest from 3.10.0 to 4.0.0 (#394)

Author: pyup-bot

requirements_dev.txt

@@ -15,7 +15,7 @@ quart==0.6.10;python_version>="3.6"  # pyup: update minor
 coverage==4.5.1  # pyup: update minor
 flake8==3.5.0  # pyup: update minor
 tox==3.5.2  # pyup: update minor
-pytest==3.10.0  # pyup: update minor
+pytest==4.0.0  # pyup: update minor
 pytest-runner==4.2  # pyup: update minor
 pytest-asyncio==0.9.0  # pyup: update minor
 pytest-tornado5==2.0.0  # pyup: update minor