Bump black from 20.8b1 to 21.4b0
Bump black from 20.8b1 to 21.4b0

Bumps [black](https://github.com/psf/black) from 20.8b1 to 21.4b0.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/master/CHANGES.md)
- [Commits](https://github.com/psf/black/commits)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
hypothesis/via,hypothesis/via,hypothesis/via