In Python 2.7, python was calling PyType_Ready() on the PyVTKClass
type in order to get its superclasses whenever hash() was called.
This raised an exception because this type predates the type/class
refinements that occurred in python 2.2.  A simple fix is to add
tp_hash to the type.

Change-Id: I9f2ec46839bd8409bbc3bfadd3a7ac8791d36ca3