Update sphinx requirement from ~=8.1.3 to ~=8.2.1

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/v8.2.1/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v8.1.3...v8.2.1)

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

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

Author: dependabot[bot]

requirements-docs.txt

@@ -11,6 +11,6 @@
 # setup.py.
 ipython ~=7.34.0
 numpydoc ~=1.7
-sphinx ~=8.1.3
+sphinx ~=8.2.1
 sphinx-copybutton ~=0.5.2
 sphinx_rtd_theme ~=3.0.2