[vtkusers] VTK Python3 support

[Fahlgren, Eric]

We'd really love to be using Py3, it's got all sorts of improvements over Py2 and more are released every few months.  Almost all of our application tools (wxPython, scipy, numpy, py2exec, to name the big ones) now have support in Py3, and VTK is the exception.  We've made sure our source is pretty much Py3 compliant and we're ready to do the final port as soon as vtk is finally updated...