Bump black from 20.8b1 to 21.4b2 (#47)

Bumps [black](https://github.com/psf/black) from 20.8b1 to 21.4b2.
- [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>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

poetry.lock

@@ -24,7 +24,7 @@ pyhumps = "^1.6.1"
 [tool.poetry.dev-dependencies]
 pytest = "^6.2"
 pre-commit = "^2.11.1"
-black = "^20.8b1"
+black = "^21.4b2"
 pylint = "^2.8.2"
 isort = "^5.8.0"
 mypy = "^0.812"