Bump black from 21.8b0 to 21.9b0

Bumps [black](https://github.com/psf/black) from 21.8b0 to 21.9b0.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](https://github.com/psf/black/commits)

---
updated-dependencies:
- dependency-name: black
  dependency-type: direct:development
...

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

Author: dependabot[bot]

poetry.lock

@@ -13,7 +13,7 @@ python = "^3.9"
 [tool.poetry.dev-dependencies]
 yamllint = "^v1.26.3"
 codespell = "^2.1.0"
-black = "^21.8b0"
+black = "^21.9b0"
 flake8 = "^3.9.2"
 pep8-naming = "^0.12.1"
 pytest = "^6.2.5"