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] <support@github.com>

requirements-dev.txt

@@ -1,6 +1,6 @@
 -r docs/requirements.txt
 django==3.2
-black==20.8b1
+black==21.4b0
 isort==5.8.0
 flake8==3.9.0
 pydocstyle==6.0.0