Bump sphinx-rtd-theme from 0.5.0 to 0.5.1 (#882)

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[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/requirements.txt

@@ -1,6 +1,6 @@
 pyenchant==3.2.0
 sphinx==3.4.3
-sphinx-rtd-theme==0.5.0
+sphinx-rtd-theme==0.5.1
 sphinxcontrib-asyncio==0.3.0
 sphinxcontrib-spelling==7.1.0
 towncrier==19.2.0