Update dependency sphinx-rtd-theme to v3

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
sphinx-rtd-theme	==2.0.0 -> ==3.0.0

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Release Notes

readthedocs/sphinx_rtd_theme (sphinx-rtd-theme)

### [`v3.0.0`](https://redirect.github.com/readthedocs/sphinx_rtd_theme/compare/2.0.0...3.0.0) [Compare Source](https://redirect.github.com/readthedocs/sphinx_rtd_theme/compare/2.0.0...3.0.0)

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