"< 0" comparison is always needed, not only in CPython and PyPy (in case we ever support more)

Cython/Utility/TypeConversion.c

@@ -560,7 +560,7 @@ static CYTHON_INLINE {{TYPE}} {{FROM_PY_FUNCTION}}(PyObject *x) {
             if (unlikely(Py_SIZE(x) < 0)) {
                 goto raise_neg_overflow;
             }
-#elif CYTHON_COMPILING_IN_PYPY
+#else
             {
                 // misuse Py_False as a quick way to compare to a '0' int object in PyPy
                 int result = PyObject_RichCompareBool(x, Py_False, Py_LT);