Update tox to 3.15.0

[pyup-bot]

This PR updates tox from 3.14.6 to 3.15.0.

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

Links

• PyPI: https://pypi.org/project/tox
• Docs: http://tox.readthedocs.org

[codecov]

Codecov Report

Merging #334 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #334   +/-   ##
========================================
  Coverage    81.41%   81.41%           
========================================
  Files           72       72           
  Lines         2776     2776           
========================================
  Hits          2260     2260           
  Misses         516      516           

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