The easiest way would be to have the GIL behind a feature flag that defaults to 'on'. That way you avoid yet another language split and if you don't want any possibly breaking changes you simply don't do anything at all. But if you want to run with the performance gains that a GIL free CPython would give you then you will have to do some extra testing to make sure that your stuff really is bullet proof with that flag set to the 'No-GIL' position.