-r requirements.txt
-r requirements-optional.txt
tox==3.12.1
pytest==4.5.0
coverage==4.5.3

-r requirements.txt
-r requirements-optional.txt
tox==3.13.1
pytest==4.5.0
coverage==4.5.3

Update tox from 3.12.1 to 3.13.1
Update tox from 3.12.1 to 3.13.1