Primer: Rethink expected changes logic + configuration

[cooperlees]

black-primer's expected changes feature is to be used while we continue to change the evolved syntax, but I don't think it will ever go away. As new python syntax comes and we evolve over time, this will be needed.

That said, I feel it would be nice to have the in black repo config + another mechanism to check if we should, or should not expect formatting changes. I'm all ears for better ideas.

The main problems are:

• When a projects becomes sync'ed with latest black we have to manually update our configs
• We don't have a source of truth for what black version a project is using

Ideas:

(Will update as we come up with more)

• Add to pyproject.toml to live in respective projects repository
  • In here state the black version we expect no changes for
  • This then asks, do we prefer config or repo config (I prefer confg if set, otherwise repo)

[JelleZijlstra]

It would be useful for it to work like mypy-primer: format a project with master and with the PR code, and report any differences in a PR comment. That workflow has been very helpful for testing typeshed changes. It could also separately flag files that the PR crashes on.

[JelleZijlstra]

diff-shades does this better, thanks @ichard26!