Building docs fails with the latest version of sphinx
It seems like one week ago sphinx 1.7.0 was released, and the version of numpydoc included with pandas is not compatible with it (it uses the `sphinx.util.compat` which has been deprecated).

Anaconda already includes sphinx 1.7.0, which is used by default. As in `requirements_dev.txt` sphinx is specified without version, after following the instructions in the contributing documentation, building the documentation fails.

I assume that numpydoc has been modified, if included in the repository instead of being added as a dependency. So, may be a quick solution is to specify `sphinx==1.6.7` in `requirements_dev.txt`.

Happy to contribute a pull request, if you can clarify what needs to be done.