Add PyLong_Sign() public function

API: int PyLong_Sign(PyObject *obj, int *sign)

Retrieve the sign of integer object obj (0, -1 or +1 for zero,
negative or positive integer, respectively) in a variable sign.

Return 0 on success, else -1 with an exception set. This function
always succeeds if obj is a :c:type:PyLongObject or its subtype.

I would like to propose adding the API directly to the limited API, what do you think?

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