tox tests, use setup.py extra_require as tox dep source

I want to use setup.py as the authority on packages to install for testing, done with extra_requires like so:

setup(
    # ...
    extras_require={
        'test': ['pytest', ],
    },
)

Tox only appears to be capable of installing from a requirements.txt file which just implies a step of snapshotting requirements before testing (which I'm ignorant how to do automatically) or by duplicating the test dependencies into the tox file, which is all I want to avoid. One mailing list post suggested that tox.ini should be the authority on test dependencies, but I don't wish to straightjacket tox into the project that completely.