Bump black from 21.5b2 to 21.6b0

Bumps [black](https://github.com/psf/black) from 21.5b2 to 21.6b0.
- [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)

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

requirements/test.txt

@@ -4,6 +4,6 @@ pytest-cov==2.12.1
 pytest-asyncio==0.15.1
 flake8==3.9.2
 mypy==0.902
-black==21.5b2
+black==21.6b0
 isort==5.9.1
 -r ci.txt