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This is used by some optimisations for builtins that call C-API functions directly but need to convert None arguments to NULL or special integer values in order to mimic the original Python interface.

Also add and backport the CPython macros for None checks (and True/False, while we're at it):
https://docs.python.org/3/c-api/structures.html#c.Py_Is

Closes https://github.com/cython/cython/issues/4737
See https://github.com/cython/cython/issues/4706