Update sphinx requirement from ~=3.3.1 to ~=3.4.2 (#26)

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [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...v3.4.2)

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

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

docs_requirements.txt

@@ -1,2 +1,2 @@
 sphinx_rtd_theme~=0.5.0
-sphinx~=3.3.1
+sphinx~=3.4.2