Bump sphinx from 3.3.1 to 3.4.0 (#219)

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)

Signed-off-by: dependabot[bot] <support@github.com>

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

Author: dependabot[bot]

requirements-doc.txt

@@ -1,3 +1,3 @@
 -r requirements.txt
-sphinx==3.3.1
+sphinx==3.4.0
 sphinx_rtd_theme==0.5.0