Bump python-semantic-release from 9.15.1 to 9.21.0

Bumps [python-semantic-release](https://github.com/python-semantic-release/python-semantic-release) from 9.15.1 to 9.21.0.
- [Release notes](https://github.com/python-semantic-release/python-semantic-release/releases)
- [Changelog](https://github.com/python-semantic-release/python-semantic-release/blob/master/CHANGELOG.rst)
- [Commits](python-semantic-release/python-semantic-release@v9.15.1...v9.21)

---
updated-dependencies:
- dependency-name: python-semantic-release
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]