Inside of a module, it is safe to convert a function declared
    cdef int func() except *:
        ...

into

    cdef int func() except? -1

because a) we need an exception return value anyway and b) there will always be an explicit exception check afterwards. Thus, changing the function implementation itself is safe. We must exclude functions that are only declared but not implemented since we do not control their signature and it is not safe to assume a specific exception return value if it is not declared.