Update tox from 3.9.0 to 3.12.1 (#490)

Author: pyup-bot

requirements_dev.txt

@@ -14,7 +14,7 @@ quart==0.9.1;python_version>="3.7"  # pyup: update minor
 # tests
 coverage==4.5.1  # pyup: update minor
 flake8==3.7.1  # pyup: update minor
-tox==3.9.0  # pyup: update minor
+tox==3.12.1  # pyup: update minor
 pytest==4.5.0  # pyup: update minor
 pytest-runner==4.4  # pyup: update minor
 pytest-asyncio==0.10.0  # pyup: update minor