RemovedInSphinx80Warning: Use "pathlib.Path" or "os.fspath" (Add Sphinx 8 Support)

[dylanh724]

When running a sphinx-build in Sphinx 7.3.7 using exhale>=0.3.0:

C:\users\dylan\miniconda3\envs\xbe-docs\lib\site-packages\breathe\project.py:116: RemovedInSphinx80Warning: Sphinx 8 will drop support for representing paths as strings. Use "pathlib.Path" or "os.fspath" instead.
  self._default_build_dir = os.path.dirname(app.doctreedir.rstrip(os.sep))

Note: This may be an issue for breathe-doc, but I'm not entirely certain 🤔