Support PEP 519 (fspath) in wrapped SetFileName methods
Python 3.6 adds the fspath (file system path) protocol described by PEP 519. By supporting this protocol in VTK's SetFileName() methods, we could make those methods accept Path objects, in addition to strings, without requiring the user to manually convert the Path to string.
For example, Python's open() method accepts either string objects or Path objects (as of Python 3.6), and this change would make VTK's SetFileName() method behave similarly.
See support-for-pathlib for additional discussion.