Fix the custom unpickler on Python 2.x.
Fix the custom unpickler on Python 2.x.

The custom unpickler was previously only being used on newer versions
of Python and Django. However, a recent change resulted in it being
imported on Python 2.x as well, and it turns out that this wasn't
actually compatible on 2.x. We were making use of `pickle._Unpickler`
Python implementation as the parent class (Python 3.x exposes a native
implementation of the unpickler as `Unpickler`), while 2.x didn't have
this. Furthermore, we were calling `super` on it, which didn't work on
2.x in that `Unpickler` was an old-style class.

This fixes up the compatibility issues to work on both versions.

Testing Done:
Unit tests pass on Python 2.x and 3.x.

Reviewed at https://reviews.reviewboard.org/r/10559/

beanbaginc/django-evolution