gh-130931: Add PyFrame_SetBack() function

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Add tests on PyFrame_GetBack() and PyFrame_SetBack().

• Issue: Cython cannot use "pycore_frame.h" in Py3.14a4 #130931

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📚 Documentation preview 📚: https://cpython-previews--131252.org.readthedocs.build/

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The coroutine support then needs to inject frames into the current stack. For this, we currently read and write f_back:
https://github.com/cython/cython/blob/f193ba860b94a7139cffd8b0d11b7ef977aa10d3/Cython/Utility/Coroutine.c#L784-L831
https://github.com/cython/cython/blob/f193ba860b94a7139cffd8b0d11b7ef977aa10d3/Cython/Utility/Coroutine.c#L870-L897

@da-woods: Would this function PyFrame_SetBack() be useful to Cython?

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cc @markshannon

[da-woods]

It'd work for us for our coroutine usage, but from what Mark said, it sounds like there might be issues exposing it more generally.

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@markshannon is against this API: #130931 (comment). So I prefer to close my PR.