Bump sphinx from 3.4.3 to 3.5.2 in /requirements (#1839)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.4.3 to 3.5.2.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.4.3...v3.5.2)

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