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.

Unit tests pass on Python 2.x and 3.x.