feat: add black GHA for changed files

[ahuber21]

Provide a github action that runs black code formatting in PRs.
I'm trying to configure it such that only changed files are checked.

[samir-nasibli]

Thanks @ahuber21! Looks good to me. @napetrov @Alexsandruss could you please review?

[Alexsandruss]

It's not clear how black will affect code: for example, it changed license header when I manually applied it.
Using of it for only changed files will cause formatting inconsistency between changed part and old part of repo.

[ahuber21]

Well it reformats the license because it's not pep8 conform

Block comments generally apply to some (or all) code that follows them, and are indented to the same level as that code. Each line of a block comment starts with a # and a single space (unless it is indented text inside the comment).

Therefore black adds the missing whitespace.

The effect of black is well defined and according to the docs, the coding style "can be viewed as a strict subset of PEP 8."
https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html

We already have many style inconsistencies. I would prefer conserving a reasonable commit history for files that do not change often which is why I'm adding black only for changed files. I think the benefits outweigh the downsides (like more inconsistencies). Frequently used/changed files will quickly be aligned to the consistent coding style.

[ahuber21]

/intelci: run

[Alexsandruss]

/intelci: run