This adds a test decorator called @py3only

Tests using this decorator should write the entire test in the test's docstring, so that it may contain Python 3 only syntax.  On Python 2 the test will be marked skip.  On Python 3 the test body will by compiled from the docstring and executed.  Perhaps some version of this could be moved to general test utilities, but for now this is the only test module we have, I think, that could make use of this.

I also fixed up some imports.