build(deps): bump black from 21.8b0 to 21.9b0 in /.github/workflows (#163)

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:production
...

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

Author: dependabot[bot]

.github/workflows/constraints.txt

@@ -1,6 +1,6 @@
 pip<21.3
 pre-commit==2.15.0
-black==21.8b0
+black==21.9b0
 flake8==3.9.2
 reorder-python-imports==2.6.0
 websockets==10.0