CPython: Restore usage of Py_SIZE to check sign of PyLong

Cython/Utility/TypeConversion.c

@@ -502,7 +502,7 @@ static CYTHON_INLINE PyObject* {{TO_PY_FUNCTION}}({{TYPE}} value) {
         func_type value = func_value;                                     \
         if (sizeof(target_type) < sizeof(func_type)) {                    \
             if (unlikely(value != (func_type) (target_type) value)) {     \
-                func_type zero = 0;                                       \
+                const func_type zero = 0;                                 \
                 if (is_unsigned && unlikely(value < zero))                \
                     goto raise_neg_overflow;                              \
                 else                                                      \
@@ -554,10 +554,12 @@ static CYTHON_INLINE {{TYPE}} {{FROM_PY_FUNCTION}}(PyObject *x) {
                 case  0: return 0;
                 case  1: __PYX_VERIFY_RETURN_INT({{TYPE}}, digit, ((PyLongObject*)x)->ob_digit[0]);
             }
+ #endif
+#endif
+#if CYTHON_COMPILING_IN_CPYTHON
             if (unlikely(Py_SIZE(x) < 0)) {
                 goto raise_neg_overflow;
             }
- #endif
 #endif
             if (sizeof({{TYPE}}) <= sizeof(unsigned long)) {
                 __PYX_VERIFY_RETURN_INT({{TYPE}}, unsigned long, PyLong_AsUnsignedLong(x))