vtkPythonAlgorithm: support older pythons

Older Python's PyObject_GetAttrString did not take a const char*, but
instead a char*. This disallows using static strings for the attribute
name.

Change-Id: I349b14f532c34fcc7c2ac6f03e24c171847bc51e
Merge topic 'python-algorithm-old-python' into master

4b74588b vtkPythonAlgorithm: support older pythons