Add a GitHub Action that runs gallery against a list of notable projects

[ambv]

We should be running gallery against a list of notable projects in a GitHub Action on any commit and any pull request.

A commit to master

It should run black twice: using the latest released version and the current master; and diff the codebases. This will allow us to identify which commit introduces some weird and unwanted formatting behavior.

A pull request

The GitHub action should also run on all PRs too but it needs to compare current master versus current PR instead of latest released version versus current master.

Note

We cannot make the GitHub action "fail" because whether a formatting change is desired or not is in the eye of the beholder.

[cooperlees]

So I've started on this - I've wrote a tool black-primer that will run whatever black is in PATH in parallel at a level the runner can configure.

This will enable us to run with Current Master then compare with stable / latest release on CI by uninstalling the editable / TOT version and pip install black ...

Once a few more people have played with the tool I'll look to get a PR up to add this into our CI runs unless someone beats me. Docs and code itself are up and linked here.