Use native string for attr_name instead of six.text_type.

The install_requires dependency on "six==1.4.1" is a minor headache for me, as I depend on later versions of the "six" library. I end up with errors like the following when I try to run my program's console scripts:
```
pkg_resources.VersionConflict: (six 1.6.1 ([...]/python3.2/site-packages), Requirement.parse('six==1.4.1'))
```

I was originally going to change this dependency to ``six>=1.4.1``, but then I saw that you didn't really need the ``six`` library at all: it is used only in one place to prepare ``attr_name`` for use in ``getattr()``/``setattr()``. When it comes to object attributes, Python 2.x only supports byte strings (silently encoding Unicode strings as ASCII) and Python 3.x only supports Unicode strings. Since these happen to match the native string type for both Python 2.x and Python 3.x, you don't need or want ``six.text_type`` for this.