Bump black from 19.10b0 to 20.8b1

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>

requirements-dev.txt

@@ -1,4 +1,4 @@
-black==19.10b0
+black==20.8b1
 coveralls==2.1.2
 ddt==1.4.1
 flake8-import-order==0.18.1