Bump black from 21.7b0 to 21.8b0 (#619)

Bumps [black](https://github.com/psf/black) from 21.7b0 to 21.8b0.
- [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
...

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

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

requirements/test.txt

@@ -4,6 +4,6 @@ pytest-cov==2.12.1
 pytest-asyncio==0.15.1
 flake8==3.9.2
 mypy==0.910
-black==21.7b0
+black==21.8b0
 isort==5.9.2
 -r ci.txt