Add "lint" stage in pipeline

What does this MR do and why?

Resolve https://jihulab.com/gitlab-cn/gitlab/-/issues/3254

在 Pipeline 中增加对代码格式的检查。使用 Black 和 Flake8 两个格式检查工具。

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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