Add black to workflow

[gcattan]

@qbarthelemy @sylvchev
There is an issue on pyriemann repo about using black to auto format the code.
I wonder if we could tried this here. What do you think?

name: black-action
on: [push, pull_request]
jobs:
  linter_name:
    name: runner / black
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - name: Check files using the black formatter
        uses: rickstaa/action-black@v1
        id: action_black
        with:
          black_args: "."
      - name: Create Pull Request
        if: steps.action_black.outputs.is_formatted == 'true'
        uses: peter-evans/create-pull-request@v3
        with:
          token: ${{ secrets.GITHUB_TOKEN }}
          title: "Format Python code with psf/black push"
          commit-message: ":art: Format Python code with psf/black"
          body: |
            There appear to be some python formatting errors in ${{ github.sha }}. This pull request
            uses the [psf/black](https://github.com/psf/black) formatter to fix these issues.
          base: ${{ github.head_ref }} # Creates pull request onto pull request or commit branch
          branch: actions/black

[gcattan]

@toncho FYI

[sylvchev]

Yes, we speak about this. It is possible to convert the repo to use black formatting, the major drawback is that it messes up with the history of commits and that tools like git blame are obscured afterwards.
What is you take on this @qbarthelemy?

[gcattan]

In the example above, when we will open a PR, the action will open a new pull request against the branch on which is based the initial PR. Once reviewed, the changes will be merged into this initial PR and included in the commit history of the PR.

[sylvchev]

Thanks for the explanation. This action seems a little bit complicated, especially for a tool like black that is made to be automatic and straightforward. If we want to use black, it is better to simply use it in pre-commit or as a linting test (as proposed in the official doc)

[gcattan]

A pre-commit hook could be complicated to share because they are not included into version control (except may be with Husky).
The example runs two actions in two different steps: one for black, the other to create a pull request. It is documented here:
https://github.com/rickstaa/action-black/
An a live example in my sample repository: gcattan/git-quality-check#11
That said, we could keep it simple, and just push black formatting as a commit inside the same branch of the PR.