chore(deps-dev): bump sphinx from 3.3.1 to 3.4.0

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-preview[bot] <support@dependabot.com>

dev-requirements.txt

@@ -5,7 +5,7 @@ pytest==4.6.9;python_version<'3.0'
 pytest==6.2.1;python_version>='3.6'
 pytest-cov==2.10.1
 Sphinx==1.8.5;python_version<'3.0'
-Sphinx==3.3.1;python_version>='3.6'
+Sphinx==3.4.0;python_version>='3.6'
 sphinx-bootstrap-theme==0.7.1
 sphinxcontrib-fulltoc==1.2.0
 sphinxcontrib-websupport==1.1.2;python_version<'3.0'