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/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v0.1.61611...v8.0.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/requirements.txt

@@ -1,4 +1,4 @@
-sphinx<8
+sphinx<9
 sphinxcontrib-bibtex
 sphinxcontrib-spelling
 autodoc_pydantic

setup.py

@@ -84,7 +84,7 @@ def run_tests(self):
     extras_require={
         "docs": [
             "autodoc_pydantic",
-            "sphinx<8",
+            "sphinx<9",
             "sphinx_design",
             "sphinx_rtd_theme",
             "sphinxcontrib-bibtex",