[3.13] gh-120642: Move _PyCode_CODE() to the internal C API (GH-121644) (#121729)

gh-120642: Move _PyCode_CODE() to the internal C API (GH-121644)

Move _PyCode_CODE() and _PyCode_NBYTES() macros to the internal C API
since they use _Py_CODEUNIT which is only part of the internal C API.
(cherry picked from commit a2bec77d25b11f50362a7117223f6d1d5029a909)

Co-authored-by: Victor Stinner <vstinner@python.org>