Fix python 3 syntax error

[nikolas]

No description provided.

[ivancrneto]

hey @nikolas, I'm working on a new fork of pymox, happy to accept your PR there: https://github.com/ivancrneto/pymox

[sgpy]

Python 3 issues

@ivancrneto is your fork for pymox working on py3.7? I am trying to port lettuce to py3 (@nikolas if you have a port ready I can use that instead)

Issues

Couple of assertion errors (like the ones mention in the PR from @nikolas)

The imports from types module, is a bit of a road block for me.