Fix result of os.walk with a path-like top directory

[mrbean-bremen]

• used to return a Path object for the top dir instead of a string
• fixes FakeOsModule.walk return a (Path, list, list) generator instead of a (str, list, list) when a pathlib object is used as input #915

Tasks

• Unit tests added that reproduce the issue or prove feature is working
• Fix or feature added
• Entry to release notes added
• Pre-commit CI shows no errors
• Unit tests passing