Outdated unstable black version 20.8b1 used in CI

[jensens]

I recommend to update to a latest stable version, in beta series there were often changes in the formatting and the version does not reflect what is used in Plone Core elsewhere (where applied).
Also its a PITA when working with the code base if some outdated version is required.

[tisto]

@mauritsvanrees @jensens do we have a place where we define the current version of black that we are using?

@jensens prio "high" for code analysis? :)

[tisto]

Fix: #1427

[jensens]

Well, high, because it really hurts while developing for Restapi in coredev. But it's merged, thanks!