Bump black from 19.10b0 to 21.6b0

Bumps [black](https://github.com/psf/black) from 19.10b0 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:production
...

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

requirements_test.txt

@@ -1,4 +1,4 @@
-black==19.10b0
+black==21.6b0
 coverage==5.5
 flake8==3.9.1
 flake8-docstrings==1.6.0