Message 200837 - Python tracker

I haven't fully thought this one through, but perhaps we could:
- deprecate calling the current concrete APIs with subclasses (since doing
so may break subclass invariants)
- add "PyBaseDict_*" APIs (etc) that subclass implementations can call.
- offer a compatibility header for earlier versions that #defines the new
APIs back to the old ones

This would be 3.5 PEP territory, though.