chore(deps): update sphinx requirement (#149)

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@v1.2.1...v4.1.0)

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

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

test-requirements.txt

@@ -10,4 +10,4 @@ pytest-cov
 pytest-xdist
 randomize>=0.13
 recommonmark
-sphinx>=1.2.1,!=1.3b1,<4.1 # BSD
+sphinx>=1.2.1,!=1.3b1,<4.2 # BSD