dev-python/pytest-xdist: Add py dependency and fix '~x64' -> '~x86'

p.s. If you're interested, I've recently started maintaining packages
in my overlay using [separate branches][1] and either [submodules][2]
or [subtree merges][3].  If we were both doing that, it would be
easier to collaborate because you could just merge my
[dev-python/pytest-xdist branch][1] and I could merge your future
changes without using filter-branch and/or cherry-pick ;).

I've also setup the test suite for the live package in my overlay, but
it's currently broken due to a [Setuptools][4] [bug][5].

[1]: http://git.tremily.us/?p=wtk-overlay.git;a=shortlog;h=refs/heads/dev-python/pytest-xdist
[2]: http://git.tremily.us/?p=wtk-overlay.git;a=shortlog;h=refs/heads/modular
[3]: http://git.tremily.us/?p=wtk-overlay.git;a=log;h=refs/heads/master
[4]: https://bitbucket.org/pypa/setuptools/pull-request/85/egg_info-search-egg-base-for-files-to-add/diff
[5]: https://bugs.gentoo.org/show_bug.cgi?id=524322