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

[codecov]

Codecov Report

Merging #493 (58fd10c) into develop (07b23de) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #493   +/-   ##
========================================
  Coverage    78.47%   78.47%           
========================================
  Files           83       83           
  Lines         3122     3122           
========================================
  Hits          2450     2450           
  Misses         672      672           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 07b23de...58fd10c. Read the comment docs.