Bump black from 20.8b1 to 21.6b0 (#126)

Bumps [black](https://github.com/psf/black) from 20.8b1 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
...

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

requirements_lint.txt

@@ -1,4 +1,4 @@
-black==20.8b1
+black==21.6b0
 flake8==3.9.2
 mypy==0.910
 pydocstyle==6.1.1