gh-112075: Remove _PyObject_SetManagedDict() function

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Move it to the internal C API and no longer export it.

• Issue: Make dict objects thread-safe in --disable-gil builds #112075

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The _PyObject_SetManagedDict() function is private, not documented, and not tested.

It's used by _PyObject_SetDict() which belongs to the internal C API and is not exported.

A code search in PyPI top 15,000 projects did not find any project using it.

The function was added by commit 8b541c0.

@DinoV: Did you add this function to the public C API on purpose?

cc @colesbury who modified the function.