pip prod(deps): update sphinx requirement from <8 to <9

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/v8.1.3/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v0.1.61611...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

pyproject.toml

@@ -35,7 +35,7 @@ setuptools = "*"
 anndata = { optional = true, version = ">=0.7.6,<0.11.2" }
 black = { optional = true, version = "*" }
 pytest-cov = { optional = true, version = "*" }
-sphinx = { optional = true, version = "<8" }                        # see issue at https://github.com/readthedocs/readthedocs.org/issues/10279
+sphinx = { optional = true, version = "<9" }                        # see issue at https://github.com/readthedocs/readthedocs.org/issues/10279
 nbsphinx = { optional = true, version = "<=0.9.5" }
 sphinx-autodoc-typehints = { optional = true, version = "<=2.3.0" }
 sphinx_rtd_theme = { optional = true, version = "<=3.0.2" }