restrict an optimisation to Py3 that is not 100% semantically correct in Py2

Cython/Utility/Exceptions.c

@@ -34,9 +34,12 @@ static CYTHON_INLINE int __Pyx_PyErr_ExceptionMatchesInState(PyThreadState* tsta
 static int __Pyx_PyErr_ExceptionMatchesTuple(PyObject *exc_type, PyObject *tuple) {
     Py_ssize_t i, n;
     n = PyTuple_GET_SIZE(tuple);
+#if PY_MAJOR_VERSION >= 3
+    // the tighter subtype checking in Py3 allows faster out-of-order comparison
     for (i=0; i<n; i++) {
         if (exc_type == PyTuple_GET_ITEM(tuple, i)) return 1;
     }
+#endif
     for (i=0; i<n; i++) {
         if (__Pyx_PyErr_GivenExceptionMatches(exc_type, PyTuple_GET_ITEM(tuple, i))) return 1;
     }