Merge #3

3: Pin sphinx_rtd_theme to latest version 0.5.1 r=aragilar a=pyup-bot


This PR pins [sphinx_rtd_theme](https://pypi.org/project/sphinx_rtd_theme) to the latest release **0.5.1**.



*The bot wasn't able to find a changelog for this release. [Got an idea?](https://github.com/pyupio/changelogs/issues/new)*

<details>
  <summary>Links</summary>
  
  - PyPI: https://pypi.org/project/sphinx-rtd-theme
  - Repo: https://github.com/readthedocs/sphinx_rtd_theme
</details>



Co-authored-by: pyup-bot <github-bot@pyup.io>

Author: bors[bot]

doc-requirements.txt

@@ -1,4 +1,4 @@
 sphinx>=1.3
 sphinxcontrib-napoleon>=0.4.3
 mock
-sphinx_rtd_theme
+sphinx_rtd_theme==0.5.1