'PosixPath' object has no attribute 'rstrip'

[mo271]

Describe the bug

FAILED: rtd/nonexistent /home/runner/work/libjxl/libjxl/build/rtd/nonexistent 
cd /home/runner/work/libjxl/libjxl/build && /home/runner/.local/bin/sphinx-build -q -W -b html -j auto /home/runner/work/libjxl/libjxl/doc/sphinx /home/runner/work/libjxl/libjxl/build/rtd

Exception occurred:
  File "/home/runner/.local/lib/python3.10/site-packages/breathe/project.py", line 1[16](https://github.com/libjxl/libjxl/actions/runs/5950747805/job/16139145864?pr=2745#step:18:17), in __init__
    self._default_build_dir = os.path.dirname(app.doctreedir.rstrip(os.sep))
AttributeError: 'PosixPath' object has no attribute 'rstrip'

How to Reproduce

see the failing workflow here:
https://github.com/libjxl/libjxl/actions/runs/5950747805/job/16139145864?pr=2745

Environment Information

[It is a bit of a hassle to produce the output and also to attach error log `/tmp/sphinx-err-kpqgavu2.log` as suggested, because this is happening in the github ci. I open this nonetheless, because I hope that the little information that is provided already might be helpful...]

Sphinx extensions

No response

Additional context

No response

[AA-Turner]

See breathe-doc/breathe#943