Update tox to 3.21.3

[pyup-bot]

This PR updates tox from 3.21.1 to 3.21.3.

Changelog

3.21.3

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

Bugfixes
^^^^^^^^

- Fix a killed tox (via SIGTERM) leaving the commands subprocesses running
by handling it as if it were a KeyboardInterrupt - by :user:`dajose`
`1772 <https://github.com/tox-dev/tox/issues/1772>`_

3.21.2

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

Bugfixes
^^^^^^^^

- Newer coverage tools update the ``COV_CORE_CONTEXT`` environment variable, add it to the list of environment variables
that can change in our pytest plugin - by :user:`gaborbernat`.
`1854 <https://github.com/tox-dev/tox/issues/1854>`_

Links

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

[coveralls]

Coverage remained the same at 86.667% when pulling 1ed4abd on pyup-update-tox-3.21.1-to-3.21.3 into b73c34e on master.

[pyup-bot]

Closing this in favor of #249