Bump black from 19.10b0 to 21.6b0

Bumps [black](https://github.com/psf/black) from 19.10b0 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] <[email protected]>

Author: dependabot-preview[bot]