> This change goes directly against PEP 387.

The change respects the documentation which always documented the result type as "void".

3.10: https://docs.python.org/dev/c-api/tuple.html#c.PyTuple_SET_ITEM
3.5: https://docs.python.org/3.5/c-api/tuple.html#c.PyTuple_SET_ITEM
2.7: https://docs.python.org/2.7/c-api/tuple.html#c.PyTuple_SET_ITEM

This change is backward incompatible on purpose: it's to implement the documented behavior, and the macro was misused leading to a bug: "PyList_SET_ITEM (l, i, obj) < 0" in py-qt4.

I would also prefer a deprecation warning, but I don't see how do detect when a macro is abused to write "x = SET();" or "SET() = x;" Maybe a C linter could detect that, but I don't know any tool doing that.

The best we can do is to announce the change. That's why I documented in What's New in Python 3.10, and not only "hidden" in the Changelog:
https://docs.python.org/dev/whatsnew/3.10.html#id2

My expectation is that apart py-qt4, no project abuse these 3 macros.