Allow specifying Black version to use in the GitHub action

[akaihola]

The Darker GitHub action currently always installs the newest Black version supported by the Darker version chosen using the version: option.

As requested by @k-dominik in #581, it would be useful to be able to choose a different Black version in certain situations.

There's already a way to choose linter versions, e.g.:

          lint: "flake8,pylint==2.13.1"

The lint: option only accepts names and optional versions of supported linters, so it can't be used to limit the Black version.

There are two obvious options for the design of this feature:

1. a separate black-version: option
2. a constraints: option which would also replace optional versions specifiers in lint:

Note that linting support is being moved into the separate Graylint tool, with Darker still supporting --lint at least in all 2.x.x versions. After that, it remains to be decided whether a unified GitHub Action for Darker and Graylint makes sense, or whether we would require users to run a Darker action and a Graylint action separately.