Fix 'Labels' class in Python 3 (fixes unit test)

Creation of an Enum with a Labels class was failing under Python 3.4.

Unit tests pass for:
* Python 3.4 Django 1.6
* Python 3.3 Django 1.5, 1.6
* Python 2.7 Django 1.5, 1.6

Tests were already failing for Django 1.4.