build(deps-dev): bump black from 21.7b0 to 21.8b0

Bumps [black](https://github.com/psf/black) from 21.7b0 to 21.8b0.
- [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

@@ -71,7 +71,7 @@ setuptools.setup(
             "pytest-instafail==0.4.2",
             "pytest-cov==2.12.1",
             "pycodestyle==2.7.0",
-            "black==21.7b0",
+            "black==21.8b0",
             "isort==5.9.3",
             "mypy==0.910",
             "psutil==5.8.0",