Update tox to 3.1.3

[pyup-bot]

This PR updates tox from 3.1.1 to 3.1.3.

Changelog

3.1.2

------------------

Bugfixes
^^^^^^^^

- Revert "Fix bug with incorrectly defactorized dependencies (`772 <https://github.com/tox-dev/tox/issues/772>`_)" due to a regression (`(799) <https://github.com/tox-dev/tox/issues/899>`_) - by :user:`obestwalter`

Links

• PyPI: https://pypi.org/project/tox
• Changelog: https://pyup.io/changelogs/tox/
• Docs: https://tox.readthedocs.org/

[pyup-bot]

Closing this in favor of #10