Bump sphinx-rtd-theme from 0.5.0 to 0.5.1

Bumps [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) from 0.5.0 to 0.5.1.
- [Release notes](https://github.com/readthedocs/sphinx_rtd_theme/releases)
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@0.5.0...0.5.1)

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

requirements-dev.txt

@@ -2,4 +2,4 @@ pre-commit==2.9.3
 pytest==5.4.3
 pytest-asyncio==0.12.0
 sphinx==3.4.3
-sphinx_rtd_theme==0.5.0
+sphinx_rtd_theme==0.5.1