Support frame eval mode in Python 3.9

[fabioz]

It seems that:

PyInterpreterState.eval_frame (PEP 523) now requires a new mandatory tstate parameter (PyThreadState*). (Contributed by Victor Stinner in :issue:38500.)

[fabioz]

As a note, I'm working on this now.

[fabioz]

@karthiknadig @int19h

I still haven't finished this... the actual fix is simple (mostly dealing with the new frame object required in _PyEval_EvalFrameDefault), but I couldn't figure out a way to do the the processing without regenerating the .c files (i.e.: having a single .c file valid for both Python 3.8 and Python 3.9).

i.e.: If the pre-processing is done on the cython level the related .c files aren't valid for all versions and I couldn't find a way to do the needed #if pre-processing on the C-level from cython.

Given that, I'm thinking about no longer commiting the .c files to the repository and always regenerating them from the .pyx files. I think that the main change is that cython will need to be available for doing the compilation (whereas right now only a C/C++ compiler is needed).

Do you think this is a problem?

-- another option could be pre-creating one .c for Python 3.8 and another for Python 3.9 (the issues here is that the related cython file which has around 600 lines becomes 20k lines of C and some more logic will be needed due to those name changes).

[fabioz]

Actually, by using some code generation and macros I've been able to make it compile with a single .c for Python 3.8 and Python 3.9 -- cython makes it a bit tricky but I think it's still reasonable (so, it should keep on working as it already does).