Bump sphinx-rtd-theme from 2.0.0 to 3.0.0 (#1142)

Bump sphinx-rtd-theme from 2.0.0 to 3.0.0 (#1142)

Bumps [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) from 2.0.0 to 3.0.0.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](https://github.com/readthedocs/sphinx_rtd_theme/compare/2.0.0...3.0.0)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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