Bump mkdocstrings from 0.20.0 to 0.21.2

Bumps [mkdocstrings](https://github.com/mkdocstrings/mkdocstrings) from 0.20.0 to 0.21.2.
- [Release notes](https://github.com/mkdocstrings/mkdocstrings/releases)
- [Changelog](https://github.com/mkdocstrings/mkdocstrings/blob/master/CHANGELOG.md)
- [Commits](https://github.com/mkdocstrings/mkdocstrings/compare/0.20.0...0.21.2)

---
updated-dependencies:
- dependency-name: mkdocstrings
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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