Test future with black --preview

[akaihola]

Black has this option:

  --preview                       Enable potentially disruptive style changes
                                  that may be added to Black's main
                                  functionality in the next major release.

We currently future-proof by running Black from its main branch in CI. This will reveal style changes once those changes to the Black major release appear in main. See:

• CI test against Black master, release process to guard against compatibliity breakage #430

Might we get an even earlier warning if we add --preview?

On the other hand, do we care? How Black reformats Python code shouldn't affect whether Darker works or not, just the output the user is going to see. There's most probably nothing to fix in Darker when Black's style changes.