Update tox to 3.2.1

[pyup-bot]

This PR updates tox from 3.1.3 to 3.2.1.

Changelog

3.2.0

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

Features
^^^^^^^^

- Switch pip invocations to use the module ``-m pip`` instead of direct invocation. This could help
avoid some of the shebang limitations.  - by :user:`gaborbernat` (`935 <https://github.com/tox-dev/tox/issues/935>`_)
- Ability to specify package requirements for the tox run via the ``tox.ini`` (``tox`` section under key ``requires`` - PEP-508 style): can be used to specify both plugin requirements or build dependencies. - by :user:`gaborbernat` (`783 <https://github.com/tox-dev/tox/issues/783>`_)
- Allow to run multiple tox instances in parallel by providing the
``--parallel--safe-build`` flag. - by :user:`gaborbernat` (`849 <https://github.com/tox-dev/tox/issues/849>`_)

Links

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

[pyup-bot]

Closing this in favor of #36