Drop black dependency

[akaihola]

This allows installing Darker without needing to install Black, and running e.g.

darker --formatter=none --isort
darker --formatter=none --flynt

and, once #744 has been merged, this will also work without needing to install Black:

darker --formatter=ruff

Before reviewing or merging this one:

• Review Black as a plugin #738
• Merge Black as a plugin #738
• Rebase on master
• Review Speed up parameterized tests which use Git #761
• Merge Speed up parameterized tests which use Git #761
• Rebase on master

[akaihola]

@clintonsteiner, I removed Black as a hard dependency, and added darker[black] extras instead. Documentation has also been updated accordingly. The default formatter is still Black, and a helpful error message is displayed if Black is needed but not installed.

The one remaining pyupgrade warning I'd like to leave as is, since it's nice to have some "dogfooding" material for when Darker gets proper pyupgrade support (see #755).