Use py.test instead of nose.

Why?

* nose is doing bad stuff to stdout/stderr, including discarding
  or showing when not needed.
* our nose test outputs contain a lot of garbage.
* nose’s output is much harder to debug.
* nose is generally disliked, while py.test is the “state-of-the-art”
  solution in the Python world.

Note that there are some bugs that kinda break it for Nikola (and
that’s basically why we run it twice, once for doctests and once for
the regular test suite)

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>