AttributeError in _find_pkg_data_path with Python 3.2

Using `get_pkg_data_filename` fails for me in one case on Python 3.2 only (works on 2.7 and 3.3) like this:
```
In [2]: tevpy.data.poisson_stats_image()
ERROR: AttributeError: 'NoneType' object has no attribute 'split' [astropy.utils.data]
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-2-c050b9fcebb1> in <module>()
----> 1 tevpy.data.poisson_stats_image()

/Users/deil/Library/Python/3.2/lib/python/site-packages/tevpy-0.1-py3.2-macosx-10.8-x86_64.egg/tevpy/data/__init__.py in poisson_stats_image(extra_info)
     70     else:
     71         filename = 'poisson_stats_image/counts.fits.gz'
---> 72         out = fits.getdata(get_pkg_data_filename(filename))
     73 
     74     return out

/opt/local/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packages/astropy/utils/data.py in get_pkg_data_filename(data_name)
    451             return hashfn
    452     else:
--> 453         datafn = _find_pkg_data_path(data_name)
    454         if os.path.isdir(datafn):
    455             raise IOError("Tried to access a data file that's actually "

/opt/local/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packages/astropy/utils/data.py in _find_pkg_data_path(data_name)
    691         # not called from inside an astropy package.  So just pass name through
    692         return data_name
--> 693     rootpkgname = module.__package__.split('.')[0]
    694 
    695     rootpkg = __import__(rootpkgname)

AttributeError: 'NoneType' object has no attribute 'split'
```
Example is here:
https://travis-ci.org/gammapy/tevpy/jobs/9024931#L459

@iguananaut @eteq This is a continuation of the discussion here:
https://github.com/astropy/astropy/issues/1241#issuecomment-20906803