Sphinx 9 compatibility

ReadTheDocs CI stopped working with the release of Sphinx 9:

https://app.readthedocs.org/projects/distributed/builds/31215915/
```
Traceback (most recent call last):
File "/home/docs/checkouts/readthedocs.org/user_builds/distributed/envs/stable/lib/python3.11/site-packages/sphinx/config.py", line 597, in eval_config_file
exec(code, namespace) # NoQA: S102
^^^^^^^^^^^^^^^^^^^^^
File "/home/docs/checkouts/readthedocs.org/user_builds/distributed/checkouts/stable/docs/source/conf.py", line 10, in
from sphinx.ext.autosummary import Autosummary, get_documenter
ImportError: cannot import name 'get_documenter' from 'sphinx.ext.autosummary' (/home/docs/checkouts/readthedocs.org/user_builds/distributed/envs/stable/lib/python3.11/site-packages/sphinx/ext/autosummary/__init__.py)
```

I'm pinning sphinx 8 for the time being.