Use unittest instead of weird pytest.

Since Travis mentions having pytest support, I was confused about why
all builds were failing with a "pytest not found" error. I eventually
realized that my dev virtualenv contains Logilab's "pytest" [0] instead
of "pytest" [1].

This commit should have a passing build (fingers crossed).

[0]: Part of the logilab-common module: https://bitbucket.org/logilab/logilab-common
[1]: https://pypi.python.org/pypi/pytest