Update sphinx-rtd-theme requirement from >=1.0,<1.4 to >=1.0,<2.1

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...2.0.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

pyproject.toml

@@ -24,7 +24,7 @@ python = ">=3.8"
 
 [tool.poetry.group.docs.dependencies]
 sphinx = ">=4.5,<7.2"
-sphinx_rtd_theme = ">=1.0,<1.4"
+sphinx_rtd_theme = ">=1.0,<2.1"
 
 [tool.poetry.group.dev.dependencies]
 coverage = {extras = ["toml"], version = ">=5.1,<8.0"}