Allow subtypes of a cypclass wrapper to be converted to that cypclass base

Cython/Utility/CyObjects.cpp

@@ -208,7 +208,7 @@
     template <typename U>
     static inline U* __Pyx_PyObject_AsCyObject(PyObject * ob, PyTypeObject * type) {
         // the PyObject is not of the expected type
-        if (ob->ob_type != type) {
+        if ( !PyType_IsSubtype(ob->ob_type, type) ) {
             PyErr_Format(PyExc_TypeError, "Cannot convert PyObject %s to CyObject %s", ob->ob_type->tp_name, type->tp_name);
             return NULL;
         }