When running `tox` command without specifying any `-e` option, or when no $TOXENV environment variable is specified, the 'envlist' in tox.ini file determines the list of environment tox will operate on.
http://tox.readthedocs.io/en/latest/config.html#confval-envlist=CSV
I chose Python 2.7 and 3.5 because I assume they are the versions that other fonttols developers run their tests on locally, before pushing and let the CI run the tests on the rest of the pythons.
The glob pattern determines which classes are considered for test collection by pytest.
We already follow this pattern for the unittest.TestCase derived subclasses
(which are collected regardless of this option).
the glob pattern will restricts the search only for pytest's kind of tests; it has no effect on methods of unittest.TestCase derived class, since pytest still uses unittest's own collection framework to collect those.
The 'minversion' is required to support some of the latest advanced features.
The current py.test is 2.8.7. You can update it with `pip install --upgrade pytest`.
