Python SystemError fix with -builtin

Fix error when append on a SWIG Object with -builtin:

  x.append(10)
  SystemError: error return without exception set

Having append and next methods on a SWIG object by default doesn't seem
right to me though.

Lib/python/pyrun.swg

@@ -569,6 +569,7 @@ SwigPyObject_append(PyObject* v, PyObject* next)
   next = tmp;
 #endif
   if (!SwigPyObject_Check(next)) {
+    PyErr_SetString(PyExc_TypeError, "Attempt to append a non SwigPyObject");
     return NULL;
   }
   sobj->next = next;