Bump sphinx-autobuild from 2020.9.1 to 2021.3.14

Bumps [sphinx-autobuild](https://github.com/executablebooks/sphinx-autobuild) from 2020.9.1 to 2021.3.14.
- [Release notes](https://github.com/executablebooks/sphinx-autobuild/releases)
- [Changelog](https://github.com/executablebooks/sphinx-autobuild/blob/main/NEWS.rst)
- [Commits](https://github.com/executablebooks/sphinx-autobuild/commits

)

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