Bump black from 19.10b0 to 20.8b1

Bumps [black](https://github.com/psf/black) from 19.10b0 to 20.8b1.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/master/CHANGES.md)
- [Commits](https://github.com/psf/black/commits)

sphinx-doc/requirements_sphinx.txt

@@ -3,7 +3,7 @@ appdirs==1.4.4
 astor==0.8.1
 attrs==19.3.0
 Babel==2.8.0
-black==19.10b0
+black==20.8b1
 certifi==2020.6.20
 chardet==3.0.4
 Click==7.0