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@v0.1.61611...v7.2.6)

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

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

setup.cfg

@@ -40,7 +40,7 @@ test =
     pytest-astropy-header
     pytest-doctestplus
 docs =
-    sphinx<7
+    sphinx<8
     numpydoc
     sphinx_rtd_theme