Re-implement magic trailing comma handling

[ambv]

Re-implement magic trailing comma handling:

• when a trailing comma is specified in any bracket pair, that signals to Black that this bracket pair needs to be always exploded, e.g. presented as "one item per line";

• this causes some changes to previously formatted code that erroneously left trailing commas embedded into single-line expressions;

• internally, Black needs to be able to identify trailing commas that it put itself compared to pre-existing trailing commas. We do this by using/abusing lib2to3's was_checked attribute. It's True for internally generated trailing commas and False for pre-existing ones (in fact, for all pre-existing leaves and nodes).

Fixes #1288

Thank you, Sentry, for sponsoring this effort.

[ambv]

Primer will obviously fail due to expected formatting changes. We'll be dealing with the other failures here, looks like they're infrastructure failures.

[ambv]

LOL, such a rookie mistake. Will fix momentarily.

[zsol]

This looks great! Much clearer logic than before. I spot checked some of the primer changes and they look good too

[ichard26]

I'm curious on why the auto generated documentation files from the README are included with your "fix dealing with generated files in docs" commit. Is it a mistake or am I missing something?

[ambv]

Those files were always committed. Look at the history of black/docs/authors.md. A few of the new ones were missing so I added those, too.

[zyv]

❤️ ❤️ ❤️

[ichard26]

Ah that explains it, thanks! When I look back at the history, those files like black/docs/authors.md were simlinks instead of regular markdown documents. IIRC, those simlinks were causing trouble with GitHub Actions since they were pointing at non existing files. My only concern is these files may become outdated as documentation changes roll in. Oh well, it's probably on me to remind people on updating the autogenerated files as a triager ... :)

[WhyNotHugo]

Oh well, it's probably on me to remind people on updating the autogenerated files as a triager

Might make sense to add a local pre-commit hook to rebuild them.