#174.

cc @arigo -- this is a minimal implementation for _cffi_backend.c. Questions:

• Can we remove the PY_VERSION checks? It's hard to test the Python-2 case, and as far as I can tell you only support Python 3.8+.
• Linked to the above, ideally we would consider module isolation, but this requires Python 3-only API that would mean more #if branching, so I've avoided for now.
• There's a use of single-phase init (PyModule_Create) in vengine_cpy.py, but the file is marked as deprecated. Is the file still used / should I update this?
• _my_Py_InitModule() and b_init_cffi_1_0_external_module() will be tricky to migrate. Would it be possible to use the dynamic creation functions here instead?

A