python2 support

[rrauenza]

Would you be open to supporting python 2.7?

It looks like the only issues are the calls to super(). Python2 requires the (current classname, self) to be passed, but you can just 'from builtins import (super,)' at the top.

The other issue is dictionary keys are lists in python2 so you can't do set operations on them -- You can fix this by changing the code to:

  to_constraints = set(getattr(to_model._meta, self.option_name, {}).keys())
  from_constraints = set(getattr(from_model._meta, self.option_name, {}).keys())

[rrauenza]

See Pull Request #3