dev-python/pytest-xdist-9999.ebuild: Add this (indirect) Raven dependency

The tests currently fail due to a bug in Setuptools' egg-info handling
[1] which is only partially worked around in the distutils-r1 eclass
[2].

[1]: https://bitbucket.org/pypa/setuptools/pull-request/85/egg_info-search-egg-base-for-files-to-add/diff
[2]: https://bugs.gentoo.org/show_bug.cgi?id=524322