Update sphinx requirement from <3.4.0,>=3.0.3 to >=3.0.3,<3.6.0

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](https://github.com/sphinx-doc/sphinx/commits)

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

Author: dependabot[bot]

doc-source/requirements.txt

@@ -4,7 +4,7 @@ default-values>=0.4.2
 extras-require>=0.2.0
 furo>=2020.11.19b18
 seed-intersphinx-mapping>=0.3.1
-sphinx<3.4.0,>=3.0.3
+sphinx>=3.0.3,<3.6.0
 sphinx-copybutton>=0.2.12
 sphinx-notfound-page>=0.5
 sphinx-prompt>=1.1.0