Update sphinx requirement from ~=7.3.7 to ~=8.1.3 (#186)

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@v7.3.7...v8.1.3)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements-docs.txt

@@ -11,6 +11,6 @@
 # setup.py.
 ipython ~=7.34.0
 numpydoc ~=1.7
-sphinx ~=7.3.7
+sphinx ~=8.1.3
 sphinx-copybutton ~=0.5.2
 sphinx_rtd_theme ~=3.0.1