build(deps): bump black from 23.7.0 to 23.9.1 in /lib/spack/docs (#39937)

Bumps [black](https://github.com/psf/black) from 23.7.0 to 23.9.1.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](https://github.com/psf/black/compare/23.7.0...23.9.1)

---
updated-dependencies:
- dependency-name: black
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

lib/spack/docs/requirements.txt

@@ -8,6 +8,6 @@ pygments==2.16.1
 urllib3==2.0.4
 pytest==7.4.2
 isort==5.12.0
-black==23.7.0
+black==23.9.1
 flake8==6.1.0
 mypy==1.5.1