++++++++++++++++
* Choose testenvs from ``tox.ini`` by matching factors.
* This is a slightly *backward incompatible* change
* If a Python version isn't declared in the ``tox.ini``,
it may not be run.
* Additional envs may be run if they also match the factors,
for example, ``py34-django17`` and ``py34-django18`` will
both match the default for Python 3.4 (``py34``).
* Factor matching extends to overrides set in ``tox.ini``.