Make vtkVariantStrictWeakOrder(a,b) work with Py3k.
For Python 2, this method operated like the cmp(a,b) method. In Python 3, there is no cmp(a,b) method, so instead this method now returns (a < b) like the original C++ method.
For Python 2, this method operated like the cmp(a,b) method. In Python 3, there is no cmp(a,b) method, so instead this method now returns (a < b) like the original C++ method.