Fix required css property

I feel like I'm missing something huge.

On Django 1.7, this fixes a bug I have. (The required class is not applied to my formField div.) I think that this have never worked, but this is tested.
There's a difference, but I don't know where is the bug:

https://github.com/django/django/blob/stable/1.4.x/django/utils/six.py#L629
https://github.com/django/django/blob/stable/1.7.x/django/utils/six.py#L629