Update sphinx-rtd-theme requirement from >=1.3,<3.0 to >=1.3,<4.0

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@1.3.0...3.0.1)

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

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

Author: dependabot[bot]

pyproject.toml

@@ -44,7 +44,7 @@ optional = true
 
 [tool.poetry.group.docs.dependencies]
 sphinx = ">=6,<8"
-sphinx-rtd-theme = ">=1.3,<3.0"
+sphinx-rtd-theme = ">=1.3,<4.0"
 certifi = ">=2018.4.16"
 expiringdict = ">=1.1.4"
 pyrfc3339 = ">=1.0"