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)

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

Signed-off-by: dependabot[bot] <[email protected]>

requirements-lint.txt

@@ -1,3 +1,3 @@
 -r requirements-dev.txt
-black==21.5b2
+black==21.6b0
 flake8==3.9.2