Bump black from 21.6b0 to 21.7b0

Bumps [black](https://github.com/psf/black) from 21.6b0 to 21.7b0.
- [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]

requirements.txt

@@ -1,5 +1,5 @@
 isort==5.9.2
-black==21.6b0; implementation_name=="cpython"
+black==21.7b0; implementation_name=="cpython"
 pytest==6.2.4
 pytest-asyncio==0.15.1
 pytest-cov==2.12.1