build(deps): bump pygments from 2.15.1 to 2.16.1 in /lib/spack/docs (#39365)

Bumps [pygments](https://github.com/pygments/pygments) from 2.15.1 to 2.16.1.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](https://github.com/pygments/pygments/compare/2.15.1...2.16.1)

---
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==0.5.0
 sphinx-rtd-theme==1.2.2
 python-levenshtein==0.21.1
 docutils==0.18.1
-pygments==2.15.1
+pygments==2.16.1
 urllib3==2.0.4
 pytest==7.4.0
 isort==5.12.0