[python] Fix inheriting from multiple director classes

Fix mishandling of a Python class inheriting from multiple SWIG-wrapped
director classes.

Fixes #422
Fixes https://sourceforge.net/p/swig/bugs/1379/

Lib/python/pyrun.swg

@@ -792,6 +792,7 @@ SwigPyObject_append(PyObject* v, PyObject* next)
     PyErr_SetString(PyExc_TypeError, "Attempt to append a non SwigPyObject");
     return NULL;
   }
+  ((SwigPyObject *)next)->next = sobj->next;
   sobj->next = next;
   Py_INCREF(next);
   return SWIG_Py_Void();