Migrate to pathlib.Path and Support __fspath__ in PySide

Python has begun to use pathlib.Path instead of os.path since Python 3.4 .
Since we no longer need to go back to Python 2.7 in PySide6, I propose to switch PySide6 also to use the pathlib.

After changing very much code, it became obvious that we always need to revert the effect of Path when a PySide function deals with it. That was enough motivation to work on supporting the __fspath__ attribute that will make it easy to implement Path acceptance everywhere.