Update pytest from 3.7.2 to 3.7.3

Author: pyup-bot

requirements_dev.txt

@@ -15,7 +15,7 @@ quart==0.6.5;python_version>="3.6"
 coverage==4.5.1
 flake8==3.5.0
 tox==3.2.1
-pytest==3.7.2
+pytest==3.7.3
 pytest-runner==4.2
 pytest-asyncio==0.9.0
 pytest-tornado==0.5.0