Update sphinx-rtd-theme to 0.4.0

[pyup-bot]

This PR updates sphinx-rtd-theme from 0.3.1 to 0.4.0.

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/

[coveralls]

Pull Request Test Coverage Report for Build 523

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.784%

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Totals
Change from base Build 505:	0.0%
Covered Lines:	4369
Relevant Lines:	4468

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💛 - Coveralls