Update sphinx requirement from ==7.* to ==8.*

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@v7.0.0rc1...v8.1.2)

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

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

requirements-dev.txt

@@ -11,5 +11,5 @@ wheel
 docutils == 0.20.*
 furo
 myst-parser == 2.*
-sphinx == 7.*
+sphinx == 8.*
 sphinx_autodoc_typehints == 2.*

requirements-readthedocs.txt

@@ -6,5 +6,5 @@ pyjson5 == 1.6.7
 docutils == 0.20.*
 furo
 myst-parser == 2.*
-sphinx == 7.*
+sphinx == 8.*
 sphinx_autodoc_typehints == 2.*