Message about unclosed files in affiliated packages

For a number of affiliated packages, I see the following when running ``python setup.py test`` with Python 3.4:

```
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name='/Volumes/Raptor/Library/Python/3.4/lib/python/site-packages/astropy-1.0.dev10555-py3.4-macosx-10.8-x86_64.egg/astropy/extern/bundled/six.py' mode='r' encoding='utf-8'>
```

This occurs after the tests have run. @cdeil also reported a similar issue in the past.

To reproduce:

```
git clone git@github.com:astrofrog/wcsaxes.git
cd wcsaxes
python setup.py test  # with Python 3.4
```

Output:

```
...
=============== 4 failed, 113 passed, 1 skipped in 8.23 seconds ================
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name='/Volumes/Raptor/Library/Python/3.4/lib/python/site-packages/astropy-1.0.dev10555-py3.4-macosx-10.8-x86_64.egg/astropy/extern/bundled/six.py' mode='r' encoding='utf-8'>
/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/importlib/_bootstrap.py:2150: ImportWarning: sys.meta_path is empty
sys:1: ResourceWarning: unclosed file <_io.BufferedReader name=10>
sys:1: ResourceWarning: unclosed file <_io.BufferedReader name=12>
```

I'm opening this here because I wonder if it's to do with the way we import the six module?