Re-adopt pre-commit hook with Black reformatter (or not?)

[dodo920306]

According to docs/index.md, Cello uses Black code style.

...
## [Contribute to the Project](contributing.md)
* [How to Contribute](contributing.md)
* [Code Style Guide](https://black.readthedocs.io/en/stable/the_black_code_style.html)

## Design Documentation
* [Architecture Design](design/arch.md)
...

Moreover, there is a .pre-commit-config.yaml file about this.

However, the current code here doesn't seem to be like that.

This PR is about whether to keep Black usage as recorded, or is simply using tox with flake8 in the CI pipeline enough?

To use it, you must run

$ pip install pre-commit black
$ pre-commit install

so that the hook can be applied every time you commit. (Check out https://github.com/pre-commit/pre-commit for more info.)

Optionally, you can run

$ pre-commit run --all-files

to trigger this without commit.

[dodo920306]

Please consider and answer the question before merging this.

Also, another problem is should this be integrated with the CI pipeline?

[yeasy]

I think using tox with flake8 in the CI pipeline is enough? @YoungHypo what is your favor?

[YoungHypo]

I think using tox with flake8 in the CI pipeline is enough? @YoungHypo what is your favor?

If we decide to adopt black for code formatting, adding a black check step in the CI could help us catch unformatted code early, before it gets merged. @yeasy @dodo920306

[dodo920306]

I think using tox with flake8 in the CI pipeline is enough? @YoungHypo what is your favor?

If we decide to adopt black for code formatting, adding a black check step in the CI could help us catch unformatted code early, before it gets merged. @yeasy @dodo920306

I have no opinion. If we want to do it, we have to make a new pipeline step to execute black at first before flake8 checks and then commit the changes if any.

[yeasy]

after discussion, we will take:

• pre-commit hook: black + flake8 (into make check)
• CI: flake8