Bump black from 20.8b1 to 21.12b0 (#35)

Bumps [black](https://github.com/psf/black) from 20.8b1 to 21.12b0.
- [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] <[email protected]>

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

Author: dependabot[bot]

requirements-dev.txt

@@ -1,4 +1,4 @@
-black==20.8b1
+black==21.12b0
 click==8.0.3
 coverage==4.5.4
 flake8==3.7.9