⬆️ Update sphinx-rtd-theme requirement

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@1.0.0...3.0.0)

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

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

setup.py

@@ -39,7 +39,7 @@ def _parse_requirements(path, encoding="utf-8"):
     extras_require={
         "doc": [
             "sphinx>=4.5.0,<8.0.0",
-            "sphinx_rtd_theme>=1.0,<1.5",
+            "sphinx_rtd_theme>=1.0,<3.1",
             "sphinx-markdown-tables~=0.0.15",
             "sphinx-click>=3.1,<6.1",
             "sphinx_copybutton~=0.5.0",