Adds Converter.__hash__ to fix Python 3 tests

Currently the Python 3 tests are failing because types that define `__eq__` are unhashable in Python 3, unless otherwise specified.

See https://docs.python.org/3.1/reference/datamodel.html#object.__hash__
> If a class that overrides \__eq__() needs to retain the implementation of __hash__() from a parent class, the interpreter must be told this explicitly by setting \__hash__ = \<ParentClass>.\__hash__. Otherwise the inheritance of __hash__() will be blocked, just as if __hash__ had been explicitly set to None.