Bump black from 19.10b0 to 20.8b1 in /backend

Bumps [black](https://github.com/psf/black) from 19.10b0 to 20.8b1.
- [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] <support@github.com>

Author: dependabot[bot]

backend/requirements.txt

@@ -21,7 +21,7 @@ cryptography==3.1
 flask_jwt_extended==3.24.1
 
 flake8==3.8.3
-black==19.10b0
+black==20.8b1
 pre-commit==2.7.1
 
 lorem==0.1.1