Pin sphinx_rtd_theme to latest version 0.4.3

Created by: pyup-bot

This PR pins sphinx_rtd_theme to the latest release 0.4.3.

The bot wasn't able to find a changelog for this release. Got an idea?

Links

• PyPI: https://pypi.org/project/sphinx-rtd-theme
• Repo: https://github.com/rtfd/sphinx_rtd_theme/