build(deps): bump urllib3 from 2.0.5 to 2.0.6 in /lib/spack/docs (#40286)

Bumps [urllib3](https://github.com/urllib3/urllib3) from 2.0.5 to 2.0.6.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/main/CHANGES.rst)
- [Commits](https://github.com/urllib3/urllib3/compare/v2.0.5...2.0.6)

---
updated-dependencies:
- dependency-name: urllib3
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

@@ -5,7 +5,7 @@ sphinx-rtd-theme==1.3.0
 python-levenshtein==0.21.1
 docutils==0.18.1
 pygments==2.16.1
-urllib3==2.0.5
+urllib3==2.0.6
 pytest==7.4.2
 isort==5.12.0
 black==23.9.1