Fix GCC -Wconversion warning in C utility code (GH-4854)

Cython/Utility/StringTools.c

@@ -953,7 +953,7 @@ static PyObject* __Pyx_PyUnicode_BuildFromAscii(Py_ssize_t ulength, char* chars,
                 padding = PyUnicode_FromOrdinal(padding_char);
                 if (likely(padding) && uoffset > prepend_sign + 1) {
                     PyObject *tmp;
-                    PyObject *repeat = PyInt_FromSize_t(uoffset - prepend_sign);
+                    PyObject *repeat = PyInt_FromSsize_t(uoffset - prepend_sign);
                     if (unlikely(!repeat)) goto done_or_error;
                     tmp = PyNumber_Multiply(padding, repeat);
                     Py_DECREF(repeat);