Bump black from 21.7b0 to 21.9b0 (#124)

Bumps [black](https://github.com/psf/black) from 21.7b0 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] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements_dev.txt

@@ -1,6 +1,6 @@
 -r requirements.txt
 pre-commit==2.14.0
-black==21.7b0
+black==21.9b0
 flake8==3.8.4
 isort==5.9.3
 pylint>=2.6.0