This doesn't seem like the right path - what does ``astropy_helpers/sphinx/local/python3links.inv`` alone give? I wonder whether the path should be made absolute with something like:

```
os.path.abspath(os.path.dirname(__file__), '../astropy_helpers/sphinx/local/python3links.inv')
```

otherwise it looks like if you were running ``setup.py build_sphinx`` you would go outside the package directory. Regarding hyphens, only the source directory (``astropy_helpers``) should matter. The name of the package/repo should be irrelevant because users can rename it so you shouldn't make any assumptions there.