Update sphinx requirement from ^3.3.1 to ^4.1.2

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/4.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.3.1...v4.1.2)

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

Signed-off-by: dependabot[bot] <[email protected]>

pyproject.toml

@@ -47,7 +47,7 @@ safety = "^1.9.0"
 mypy = "^0.910"
 typeguard = "^2.10.0"
 xdoctest = {extras = ["colors"], version = "^0.15.0"}
-sphinx = "^3.3.1"
+sphinx = "^4.1.2"
 sphinx-autobuild = "^2021.3.14"
 pre-commit = "^2.9.2"
 black = "^21.7b0"