Bump black from 21.4b2 to 21.6b0

Bumps [black](https://github.com/psf/black) from 21.4b2 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)

---
updated-dependencies:
- dependency-name: black
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements-test.txt

@@ -1,6 +1,6 @@
 mypy==0.902
 flake8==3.9.2
-black==21.5b2
+black==21.6b0
 
 codespell==2.1.0