Avoid casting to negative number for signedness check.

See #2186.

Cython/Utility/TypeConversion.c

@@ -696,7 +696,7 @@ static CYTHON_INLINE PyObject* {{TO_PY_FUNCTION}}({{TYPE}} value, Py_ssize_t wid
     Py_ssize_t length, ulength;
     int prepend_sign, last_one_off;
     {{TYPE}} remaining;
-    const {{TYPE}} neg_one = ({{TYPE}}) -1, const_zero = ({{TYPE}}) 0;
+    const {{TYPE}} neg_one = ({{TYPE}}) 0 - ({{TYPE}}) 1, const_zero = ({{TYPE}}) 0;
     const int is_unsigned = neg_one > const_zero;
 
     if (format_char == 'X') {