Replace testing framework from Attest to py.test

[Attest][] doesn't support Python 3.2 or higher. [py.test][] supports Python 3.x fairly well, so I suggested to replace to it to make Sider supports Python 3.x easily.


[Attest]: https://pypi.python.org/pypi/Attest
[py.test]: https://pypi.python.org/pypi/pytest/2.3.4