[issue45247] Add explicit support for Cython to the C API.

STINNER Victor added the comment: We need to add C API to abstract access to structures used by Cython: * bpo-39947: PyThreadState * bpo-40421: PyFrameObject -- nosy: +vstinner ___ Python tracker __

[issue45247] Add explicit support for Cython to the C API.

New submission from Mark Shannon : As the C API has evolved it has grown features in an ad-hoc way, driven by the needs to whoever has bothered to add the code. Maybe we should be a bit more principled about this. Specifically we should make sure that there is a well defined interface between