build(deps): bump pygments from 2.18.0 to 2.19.1 in /lib/spack/docs (#48431)

Bumps [pygments](https://github.com/pygments/pygments) from 2.18.0 to 2.19.1.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](https://github.com/pygments/pygments/compare/2.18.0...2.19.1)

---
updated-dependencies:
- dependency-name: pygments
  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

@@ -4,7 +4,7 @@ sphinx_design==0.6.1
 sphinx-rtd-theme==3.0.2
 python-levenshtein==0.26.1
 docutils==0.21.2
-pygments==2.18.0
+pygments==2.19.1
 urllib3==2.3.0
 pytest==8.3.4
 isort==5.13.2