sphinx_exercise/post_transforms.py:198: Use "pathlib.Path" or "os.fspath" instead.

### Context

FYI:

sphinx_exercise/post_transforms.py:198: RemovedInSphinx90Warning: Sphinx 9 will drop support for representing paths as strings. Use "pathlib.Path" or "os.fspath" instead.

### Proposal

_No response_

### Tasks and updates

_No response_