build(deps): bump black from 24.1.1 to 24.4.2 in /lib/spack/docs

Bumps [black](https://github.com/psf/black) from 24.1.1 to 24.4.2.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](psf/black@24.1.1...24.4.2)

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

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

lib/spack/docs/requirements.txt

@@ -8,6 +8,6 @@ pygments==2.17.2
 urllib3==2.2.0
 pytest==8.0.0
 isort==5.13.2
-black==24.1.1
+black==24.4.2
 flake8==7.0.0
 mypy==1.8.0