It is so hard to get this test to behave consistently on different
Python versions and it's not really worth it.
Compatibility with Python 3 requires with_metaclass for compatibility.
Unfortunately the implementations in python-future and six do stuff
with the stack frame that causes things to be broken with Venusian
attach. An older version of six in python-future does have a very
simple implementation that did seem to work, so I copied it into
compat.
Increase coverage of morepath.converter and mark Python 3 related
no-coverage.