Remove dependency on six and enable sphinx tests on Python 3

Just for info, this isn't a problem for ``build_sphinx`` - for which I can indeed change ``sys.path`` (I had it that way originally) but it's for these lines in ``setup_helpers.py``:

```
            ah_importer = pkgutil.get_importer('astropy_helpers')
            ah_path = os.path.abspath(ah_importer.path)
```

I couldn't figure out a way to get it to work without copying the helpers package.