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] <[email protected]>

Author: dependabot-preview[bot]

docs/requirements.txt

@@ -1,5 +1,5 @@
 marshmallow>=2.0.0rc1
 Flask==1.1.2
 sphinx==3.4.3
-sphinx-rtd-theme==0.5.0
+sphinx-rtd-theme==0.5.1
 sphinx-issues>=0.2.0