gh-89639: Revert Py_SIZE() static inline function

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Allow again to use Py_SIZE() to set an object size:
"Py_SIZE(obj) = size".

Partially revert commit cb15afc.

• Issue: [C API] PEP 674: Disallow using macros (Py_TYPE and Py_SIZE) as l-value #89639

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PR related to the discussion: https://discuss.python.org/t/pep-674-disallow-using-macros-as-l-values-and-python-3-11/18297

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Py_TYPE() and Py_SIZE() changed multiple times since 2020: https://peps.python.org/pep-0674/#py-type-and-py-size-macros

[vstinner]

I prefer to mark the PR as a draft to not merge it by mistake. I would prefer to reach a consensus and get a Steering Council decision.

[vstinner]

the Steering Council have decided not to revert these changes in 3.11 to not affect the stability of the release as at the time this was discussed, the 3.11 branch was already in release candidate 1.

ref: https://discuss.python.org/t/pep-674-disallow-using-macros-as-l-values-and-python-3-11/18297/14

I close my PR.