chore(deps): update sphinx requirement (#323)

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.rst)
- [Commits](sphinx-doc/sphinx@v1.2.1...v8.0.0rc1)

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

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

Author: dependabot[bot]

test-requirements.txt

@@ -7,5 +7,5 @@ pytest-cov
 pytest-xdist
 randomize>=0.13
 recommonmark
-sphinx>=1.2.1,!=1.3b1,<7.5 # BSD
+sphinx>=1.2.1,!=1.3b1,<8.1 # BSD
 black