Silent crash when passing a pathlib.Path object as dir to QFileDialog.getSaveFileName() using keyword arguments

The program silently crashes without error if the `dir` argument of `QFileDialog.getSaveFileName` is of type `pathlib.Path`.

The string representation of the path works fine.

I would've expected an exception for incompatible type, since silent crashes are very hard to debug.
Forgetting to convert the Path to str is something that has happened to me rather often, and it may happen to other people as well. So I believe that throwing an exception here would benefit several people.