Update tox to 3.21.1

[pyup-bot]

This PR updates tox from 3.21.0 to 3.21.1.

The bot wasn't able to find a changelog for this release. Got an idea?

Links

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

[codecov]

Codecov Report

Merging #484 (e076d2e) into develop (d1681fc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #484   +/-   ##
========================================
  Coverage    78.47%   78.47%           
========================================
  Files           83       83           
  Lines         3122     3122           
========================================
  Hits          2450     2450           
  Misses         672      672           

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