Bump sphinx-sitemap from 2.8.0 to 2.9.0 (#499)

Bumps [sphinx-sitemap](https://github.com/jdillard/sphinx-sitemap) from 2.8.0 to 2.9.0.
- [Release notes](https://github.com/jdillard/sphinx-sitemap/releases)
- [Changelog](https://github.com/jdillard/sphinx-sitemap/blob/master/CHANGELOG.rst)
- [Commits](jdillard/sphinx-sitemap@v2.8.0...v2.9.0)

---
updated-dependencies:
- dependency-name: sphinx-sitemap
  dependency-version: 2.9.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements.txt

@@ -5,7 +5,7 @@ sphinxcontrib-programoutput==0.18
 sphinx-copybutton==0.5.2
 sphinx_design==0.6.1
 sphinx-last-updated-by-git==0.3.8
-sphinx-sitemap==2.8.0
+sphinx-sitemap==2.9.0
 # fork of furo with a few changes that are not merged upstream
 git+https://github.com/haampie/furo.git@c1d161cb9e04b481ec3331db981aacfa132ecaa6#egg=furo
 python-levenshtein==0.27.1