build(deps-dev): bump blacken-docs from 1.19.1 to 1.20.0

Bumps [blacken-docs](https://github.com/asottile/blacken-docs) from 1.19.1 to 1.20.0.
- [Changelog](https://github.com/adamchainz/blacken-docs/blob/main/CHANGELOG.rst)
- [Commits](adamchainz/blacken-docs@1.19.1...1.20.0)

---
updated-dependencies:
- dependency-name: blacken-docs
  dependency-version: 1.20.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]