fix: Remove parenthesis around sole list items

[cobaltt7]

Fixes #3545, but it affects more places as well.

Description

I added this to the hug_parens_with_braces_and_square_brackets unstable style since there was a lot of overlapping code. I renamed it to concise_nested_brackets to reflect this. That is a breaking change if anyone was using --enable-unstable-feature hug_parens_with_braces_and_square_brackets, but to my knowlege, the existence of certain unstale features is not gurenteed to be stable. I can add the old name back as a (deprecated) alias or revert the rename altogether if so desired.

This does introduce more places where #4036 would be triggered:

items = [  # type: ignore
    (  # type: ignore
        {"key1": "val1", "key2": "val2", "key3": "val3"}
        if some_var == "longstring"
        else { "key": "val" }
    )
]

Checklist - did you ...

• [y] Add an entry in CHANGES.md if necessary?
• [y] Add / update tests if necessary?
• [y] Add new / update outdated documentation?

[github-actions]

diff-shades reports zero changes comparing this PR (8c4519f) to main (b677a64).

---

What is this? | Workflow run | diff-shades documentation

[JelleZijlstra]

Thanks, these changes look good.

However, I think I'd prefer to keep feature names stable through the year. The main motivation for adding --enable-unstable-feature was to allow users who want to keep some feature on to just turn on the feature in their config and have it keep working.

Also, I don't want to accept a PR that directly adds a new unstable style. I see the unstable style as a holding pen for style changes that we'd originally put into preview but later ran into issues with. New style features should go into preview instead.

[cobaltt7]

Apologies for the delay @JelleZijlstra, I've had other priorities recently.

The main issue was hug_parens_with_braces_and_square_brackets was required for the parens to be removed around multi-line expressions. I tried to identify exactly what parts it depended on, but I don't think it would be an easy refactor. I settled on just documenting this limitation instead.

I put remove_lone_list_item_parens in the unstable style due to this limitation, although it does work on its own for simple cases. If desired, I could move it to the preview style so preview users could get the benefits in those cases, or we could wait for hug_parens_with_braces_and_square_brackets. The original issue #3545 was for the more complex cases.

[cobaltt7]

Hey @JelleZijlstra just returned to this PR, sorry for letting it go stale

I removed this change from the unstable style, because it doesn't add any issues on its own, it just doesn't work in all cases without hug_parens_with_braces_and_square_brackets. I also resolved the conflicts.

Do these changes look good?

[JelleZijlstra]

Thank you!

[JelleZijlstra]

Suggested change

	- Remove parenthesis around sole list items (#4312)
	- Remove parentheses around sole list items (#4312)

[JelleZijlstra]

Add a test containing a generator expression, to make sure we don't turn it into a listcomp:

>>> [(x for x in [1])]
[<generator object <genexpr> at 0x104277400>]
>>> [x for x in [1]]
[1]

Also add tests with comments in various places (e.g., after the opening paren, before the closing paren). Such cases tend to expose bugs in Black.