Bump sphinx from 3.4.3 to 3.5.1 in /requirements (#795)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.4.3 to 3.5.1.
- [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...v3.5.1)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements/docs.txt

@@ -1 +1 @@
-Sphinx==3.4.3
+Sphinx==3.5.1