Update sphinx requirement from <4,>=3.0.3 to >=3.0.3,<8

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/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.0.3...v7.1.2)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
...

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

doc-source/requirements.txt

@@ -3,7 +3,7 @@ extras-require>=0.5.0
 furo>=2020.11.19b18
 html-section>=0.3.0
 seed-intersphinx-mapping>=1.2.2
-sphinx<4,>=3.0.3
+sphinx>=3.0.3,<8
 sphinx-copybutton>=0.2.12
 sphinx-debuginfo>=0.2.2
 sphinx-favicon>=0.2