Update sphinx-rtd-theme requirement from ~=3.0.2 to ~=3.1.0 (#83)

Updates the requirements on [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) to permit the latest version.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@3.0.2...3.1.0)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  dependency-version: 3.1.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -4,6 +4,6 @@
 Sphinx~=8.2
 doc8~=2.0.0
 reuse~=6.2.0
-sphinx-rtd-theme~=3.0.2
+sphinx-rtd-theme~=3.1.0
 sphinxcontrib-spelling~=8.0.2
 sphinx-multiversion~=0.2.4