Simplify classmethod() code in Py3 where "PyMethodDescr_Type" is directly available.

Cython/Utility/CythonFunction.c

@@ -1459,7 +1459,8 @@ static PyObject* __Pyx_Method_ClassMethod(PyObject *method) {
     // special C-API function only in Pyston and PyPy >= 5.9
     if (PyMethodDescr_Check(method))
 #else
-    // It appears that PyMethodDescr_Type is not exposed anywhere in the CPython C-API
+    #if PY_MAJOR_VERSION == 2
+    // PyMethodDescr_Type is not exposed in the CPython C-API in Py2.
     static PyTypeObject *methoddescr_type = NULL;
     if (unlikely(methoddescr_type == NULL)) {
        PyObject *meth = PyObject_GetAttrString((PyObject*)&PyList_Type, "append");
@@ -1467,6 +1468,9 @@ static PyObject* __Pyx_Method_ClassMethod(PyObject *method) {
        methoddescr_type = Py_TYPE(meth);
        Py_DECREF(meth);
     }
+    #else
+    PyTypeObject *methoddescr_type = &PyMethodDescr_Type;
+    #endif
     if (__Pyx_TypeCheck(method, methoddescr_type))
 #endif
     {