chore: 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:production
...

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

Author: dependabot[bot]

setup.py

@@ -41,7 +41,7 @@ def find_version(*file_paths):
     maintainer_email="kschwarz@hexonet.net",
     url="https://github.com/hexonet/python-sdk/",
     install_requires=[
-        "black==21.5b2",
+        "black==21.6b0",
         "docutils==0.*",
         "guzzle-sphinx-theme==0.*",
         "m2r2==0.*",