build(deps-dev): bump black from 21.5b2 to 21.6b0

Bumps [black](https://github.com/psf/black) from 21.5b2 to 21.6b0.
- [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)

---
updated-dependencies:
- dependency-name: black
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

setup.py

@@ -70,7 +70,7 @@ setuptools.setup(
             "pytest-instafail==0.4.2",
             "pytest-cov==2.12.1",
             "pycodestyle==2.7.0",
-            "black==21.5b2",
+            "black==21.6b0",
             "isort==5.8.0",
         ]
     },