build(deps): bump mypy from 1.7.1 to 1.8.0 in /lib/spack/docs (#41821)

Bumps [mypy](https://github.com/python/mypy) from 1.7.1 to 1.8.0.
- [Changelog](https://github.com/python/mypy/blob/master/CHANGELOG.md)
- [Commits](https://github.com/python/mypy/compare/v1.7.1...v1.8.0)

---
updated-dependencies:
- dependency-name: mypy
  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

@@ -10,4 +10,4 @@ pytest==7.4.3
 isort==5.13.2
 black==23.12.0
 flake8==6.1.0
-mypy==1.7.1
+mypy==1.8.0