Using `python setup.py test` is now deprecated [1], users are encouraged
to be explicit about the test command.
Running yamllint tests using the Python standard library (`unittest`)
can be done using:
python -m unittest discover
Why not nose, tox or pytest? Because they would add a dependency, make
tests running more complicated and verbose for new users, and their
benefit is not worth for this simple project (only 2 runtime
dependencies: PyYAML and pathspec).
Resolves https://github.com/adrienverge/yamllint/issues/328.
[1]: https://github.com/pypa/setuptools/pull/1878
