python3 setup.py build_sphinx fails

- Bitbucket: https://bitbucket.org/hpk42/tox/issue/289
- Originally reported by: @warsaw
- Originally created at: 2015-11-20T22:23:28.780

```
% python3 setup.py build_sphinx
running build_sphinx
error: error in 'source_dir' option: 'doc/en/' does not exist or is not a directory
```

Changing this to

```
[build_sphinx]
source-dir = doc
```

in setup.cfg fixes the problem.
