Update tox to 3.21.2

[pyup-bot]

This PR updates tox from 3.21.1 to 3.21.2.

Changelog

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

[codecov]

Codecov Report

Merging #487 (7ed7ace) into develop (07b23de) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #487   +/-   ##
========================================
  Coverage    78.47%   78.47%           
========================================
  Files           83       83           
  Lines         3122     3122           
========================================
  Hits          2450     2450           
  Misses         672      672           

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[pyup-bot]

Closing this in favor of #493