Code review comment for lp:~canonical-platform-qa/autopilot/cpo-base-classes-fix

I've already added the tests for the current implementation of _combine_base_and_extensions.

While writing them I've noticed that when two classes had the same mro the order wasn't deterministic, I've added another function for having a more fine-grained control. Currently the classes in the original __bases__ tuple have lower index than the extensions in the resulting tuple, this can be changed.

Thanks,