Fix tests to work in Python 3

[eliasdorneles]

The production code already works in Python 3, but the tests don't -- mainly because of the skinfer (which has a PY3 port in the way).

We don't really need skinfer, we only use schema generatrion to make it easy to build test fixtures.

Writing the schemas manually is a bit of a PITA, so it would be nice to find an alternative that works in all Python versions.