Fix const-cast warnings in vtkPythonInteractiveInterpreter.
PySys_GetObject/SetObject API continues to take char* (instead of const char*) till Python 3.4. Hence we need to use const_cast to avoid warnings.
PySys_GetObject/SetObject API continues to take char* (instead of const char*) till Python 3.4. Hence we need to use const_cast to avoid warnings.