For clarification, the existing behavior on master:
    When trying to raise the exception H,
        F -> G -> H -> I -> NULL
    becomes
        H -> F -> G -> NULL

    But when trying to set the exception A on top of
        B -> C -> D -> E -> C -> ...,
        it gets stuck in an infinite loop from the existing cycle.

My PR 20539 keeps the first behavior and resolves the infinite loop by making it
    A -> B -> C -> D -> E -> NULL,
    which seems consistent with the existing behavior.

So it should be strictly a bugfix. It also only changes the PyErr_SetObject code and not the PyException_SetContext code.