[Python-Dev] New shared-keys dictionary implementation (issue13903)

["Martin v. Löwis"]

> Unfortunately it seems to be the norm in CPython to publish almost
> everything in header files that get included in "Python.h".

In many cases, this is purely for historic reasons. In many additional
cases, it's to support fast access macros, at least in the interpreter
itself, but then also in extension modules.

I agree that moving the structures into the implementation is fine,
as long as there are sufficient access functions (for dictionaries,
there are plenty, of course).

Regards,
Martin