#21862: Fix TypeError in Python 3.3 when module path returns NamespacePath

This fixes the issue described in ticket [#21862](https://code.djangoproject.com/ticket/21862) where loading a custom app fails using Django 1.7a1 and Python 3.3. The issue is caused by the ``NamespacePath`` returned by ``app_module.__path__`` which is not indexable and has to be convert to a list first.

A similar solution has been applied in py.test: https://bitbucket.org/hpk42/pytest/commits/dac4900b78f2

I hope this makes sense. Let me know if there's anything that needs to be improved or added.