Update tox to 3.2.1

[pyup-bot]

This PR updates tox from 3.1.2 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>`_)

3.1.3

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

Bugfixes
^^^^^^^^

- A caching issue that caused the ``develop-inst-nodeps`` action, which
reinstalls the package under test, to always run has been resolved. The
``develop-inst-noop`` action, which, as the name suggests, is a no-op, will now
run unless there are changes to ``setup.py`` or ``setup.cfg`` files that have
not been reflected - by stephenfin (`909 <https://github.com/tox-dev/tox/issues/909>`_)


Features
^^^^^^^^

- Python version testenvs are now automatically detected instead of comparing
against a hard-coded list of supported versions.  This enables ``py38`` and
eventually ``py39`` / ``py40`` / etc. to work without requiring an upgrade to
``tox``.  As such, the following public constants are now deprecated
(and scheduled for removal in ``tox`` 4.0: ``CPYTHON_VERSION_TUPLES``,
``PYPY_VERSION_TUPLES``, ``OTHER_PYTHON_INTERPRETERS``, and ``DEFAULT_FACTORS`` -
by :user:`asottile` (`914 <https://github.com/tox-dev/tox/issues/914>`_)


Documentation
^^^^^^^^^^^^^

- Add a system overview section on the index page that explains briefly how tox works -
by :user:`gaborbernat`. (`867 <https://github.com/tox-dev/tox/issues/867>`_)

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 #380