Update pytest from 3.6.3 to 3.6.4

Author: pyup-bot

requirements_dev.txt

@@ -15,7 +15,7 @@ quart==0.6.3;python_version>="3.6"
 coverage==4.5.1
 flake8==3.5.0
 tox==3.1.2
-pytest==3.6.3
+pytest==3.6.4
 pytest-runner==4.2
 pytest-asyncio==0.8.0
 pytest-tornado==0.5.0