Message 181505 - Python tracker

Any objections to what Brett proposed?  Any preferences on a name?  How about DualImplementationTests?  Here's a patch with tests.

I added a pure_python_only() decorator in addition to accelerated_only().  I also made both work as decorators for classes and methods (just like unittest.skip()).

Finally, I added on a with_module_exported() decorator to help with the scenario from issue #16817.  Eli, I know it doesn't quite fill the gap you described in msg180669, but hopefully it's an improvement.