dev-python/pyvows: Add this (indirect) thumbor test dependency

I've submitted the find_packages sed upstream [1].  I don't know if
the package itself is compatible with Python 3, but the test suite
isn't so I'm sticking with 2.7 in PYTHON_COMPAT [2].  The test suite
also depends on gevent, because the fallback VowsSequentialRunner is
broken [3].

[1]: https://github.com/heynemann/pyvows/pull/109
[2]: https://github.com/heynemann/pyvows/issues/110#issuecomment-57243488
[3]: https://github.com/heynemann/pyvows/issues/111