Update tox to 3.22.0

[pyup-bot]

This PR updates tox from 3.21.1 to 3.22.0.

Changelog

3.22.0

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

Features
^^^^^^^^

- The value of the :conf:`requires` configuration option is now exposed via
the :class:`tox.config.Config` object - by :user:`hroncok`
`1918 <https://github.com/tox-dev/tox/issues/1918>`_

3.21.4

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

Bugfixes
^^^^^^^^

- Adapt tests not to assume the ``easy_install`` command exists, as it was removed from ``setuptools`` 52.0.0+ - by :user:`hroncok`
`1893 <https://github.com/tox-dev/tox/issues/1893>`_

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 b3fe46f on pyup-update-tox-3.21.1-to-3.22.0 into b73c34e on master.

[pyup-bot]

Closing this in favor of #259