Update tox from 3.12.1 to 3.14.0

Author: pyup-bot

requirements_dev.txt

@@ -15,7 +15,7 @@ starlette==0.12.0;python_version>="3.6"  # pyup: update minor
 # tests
 coverage==4.5.1  # pyup: update minor
 flake8==3.7.1  # pyup: update minor
-tox==3.12.1  # pyup: update minor
+tox==3.14.0  # pyup: update minor
 pytest==5.1.0  # pyup: update minor
 pytest-runner==5.1  # pyup: update minor
 pytest-asyncio==0.10.0  # pyup: update minor