tox.ini: Use py.test directly

instead of `python setup.py test`.

The advantage of using `py.test` directly is that you can now pass `py.test`
options to take advantage of all of its neat features for controlling
test collection, output control, pdb, etc. E.g.:

    tox -e py27 -- --tb=short --pdb --pyargs tests.integration.test_repos_release -k test_download

I also did some minor formatting cleanup, like removing whitespace from
ends of lines and putting spaces after commas in the envlist.