Fix vtkLoadPythonTkWidgets error with Python 2.
Fix vtkLoadPythonTkWidgets error with Python 2.

After the changes to make this module work with Python 3, it gives the
following error under Python 2.7:

UnboundLocalError: local variable 'unicode' referenced before assignment

Creating a local variable from the builtin 'unicode' fixes the issue.

keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,sumedhasingla/VTK,sumedhasingla/VTK,sumedhasingla/VTK,SimVascular/VTK,sumedhasingla/VTK,sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,sumedhasingla/VTK,keithroe/vtkoptix,keithroe/vtkoptix,sumedhasingla/VTK,sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix