build(deps-dev): bump blacken-docs from 1.16.0 to 1.18.0

Bumps [blacken-docs](https://github.com/adamchainz/blacken-docs) from 1.16.0 to 1.18.0.
- [Changelog](https://github.com/adamchainz/blacken-docs/blob/main/CHANGELOG.rst)
- [Commits](adamchainz/blacken-docs@1.16.0...1.18.0)

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

Signed-off-by: dependabot[bot] <support@github.com>