Update sphinx-rtd-theme requirement from ~=0.5.0 to ~=0.5.1 (#54)

Updates the requirements on [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) to permit the latest version.
- [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>

Author: dependabot[bot]

requirements-dev.txt

@@ -14,4 +14,4 @@ ipython ~=7.19.0
 numpydoc ~=1.1.0
 sphinx ~=3.4.3
 sphinx-copybutton ~=0.3.1
-sphinx_rtd_theme ~=0.5.0
+sphinx_rtd_theme ~=0.5.1