Bump black from 20.8b1 to 21.5b1

Bumps [black](https://github.com/psf/black) from 20.8b1 to 21.5b1.
- [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] <support@dependabot.com>

Author: dependabot-preview[bot]

poetry.lock

@@ -32,7 +32,7 @@ lesscpy = "*"
 cssmin = "*"
 jsmin = "*"
 pytest = "*"
-black = "^20.8b1"
+black = "^21.5b1"
 isort = "^5.6.4"
 
 [tool.poetry.scripts]