Bump black from 19.10b0 to 20.8b1 in /src/requirements

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-preview[bot] <support@dependabot.com>

src/requirements/dev.txt

@@ -4,7 +4,7 @@ django-debug-toolbar==2.2
 factory_boy==3.1.0
 coverage==5.2.1
 codecov==2.1.10
-black==19.10b0
+black==20.8b1
 pre-commit==2.8.1
 isort==5.6.4
 hypothesis==5.41.4