build(deps): bump black from 24.4.2 to 24.8.0 in /lib/spack/docs

Bumps [black](https://github.com/psf/black) from 24.4.2 to 24.8.0.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](psf/black@24.4.2...24.8.0)

---
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==2.18.0
 urllib3==2.2.2
 pytest==8.3.2
 isort==5.13.2
-black==24.4.2
+black==24.8.0
 flake8==7.1.0
 mypy==1.11.1