Bump sphinx from 3.3.1 to 3.4.0 (#321)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.3.1 to 3.4.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.3.1...v3.4.0)

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

requirements-dev.txt

@@ -2,5 +2,5 @@ setuptools==51.1.0.post20201221
 wheel==0.36.2
 twine==3.2.0
 unittest-xml-reporting==3.0.4
-sphinx==3.3.1
+sphinx==3.4.0
 sphinx-rtd-theme==0.5.0