By the way, I added Py_SET_REFCNT(), Py_SET_TYPE() and Py_SET_SIZE() in Python 3.9, but I left the related Py_REFCNT(), Py_TYPE() and Py_SIZE() unchanged. It remains possible to use them as l-value for example: Py_REFCNT(ob) = 1;.

I expect that few C extensions use Py_REFCNT(ob) = 1;, but maybe a few use Py_TYPE(op) = type;. For Py_SIZE(ob) = size;, Python 3.8 didn't provide any "abstract" setter function/macro, direct access to the member is also possible: ((PyVarObject*)ob)->ob_size = size;,

I chose to add new macros in Python 3.9, and maybe change the macros in Python 3.10. The change is part of a larger project:
https://github.com/vstinner/misc/blob/master/cpython/pep-opaque-c-api.rst

See bpo-39573.

No activity since May, I close the PR. I created PR #23645 which converts PyTuple_SET_ITEM() and PyList_SET_ITEM() to static inline functions.

Thanks @ZackerySpytz, I merged your PR. I wrote PR #23654 to modify also PyCell_SET().