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>

requirements-dev.txt

@@ -1,6 +1,6 @@
 -r docs/requirements.txt
 django==3.2.7
-black==21.8b0
+black==21.9b0
 isort==5.9.3
 flake8==3.9.2
 pydocstyle==6.1.1