Update tox from 3.1.2 to 3.1.3

Author: pyup-bot

requirements_dev.txt

@@ -14,7 +14,7 @@ quart==0.6.4;python_version>="3.6"
 # tests
 coverage==4.5.1
 flake8==3.5.0
-tox==3.1.2
+tox==3.1.3
 pytest==3.6.4
 pytest-runner==4.2
 pytest-asyncio==0.9.0