Replace PyTest in setup.py with make prepare like in Toil

[hannes-ucsc]

The goal was to have setup.py test install the test dependencies. Even though the goal is laudable, in practice this makes using IDEs harder (as the test dependencies are only installed for the duration of a test run) and incomplete since most of the time there are other build requirements like Sphinx that aren't covered by this.

Copy the make prepare approach from Toil and remove PyTest and tests_require from setup.py. Invoke tests directly via py.test.

[jvivian]

End of your message got cut off.

[hannes-ucsc]

Fixed it.