tox tests, use setup.py extra_require as tox deps source
<p>I want to use setup.py as the authority on packages to install for testing, done with extra_requires like so:</p>

<pre><code>setup(
    # ...
    extras_require={
        'test': ['pytest', ],
    },
)
</code></pre>

<p>Tox only appears to be capable of <a href="https://tox.readthedocs.io/en/latest/example/basic.html#depending-on-requirements-txt" rel="noreferrer">installing from a requirements.txt</a> file which just implies a step of snapshotting requirements before testing (which I'm ignorant how to do automatically) or by <a href="https://tox.readthedocs.io/en/latest/config.html#confval-deps=MULTI-LINE-LIST" rel="noreferrer">duplicating the test dependencies</a> into the tox file, which is all I want to avoid. <a href="http://lists.idyll.org/pipermail/testing-in-python/2015-March/006285.html" rel="noreferrer">One mailing list post</a> suggested that tox.ini should be the authority on test dependencies, but I don't wish to straightjacket tox into the project that completely.</p>