add option: allow_block_newline

[mathause]

Description

Checklist - did you ...

• Potentially closes Allow a single newline after opening blocks, possibly via an option setting #3551
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

This is an attempt to close #3551 via an option. This is certainly a WIP but I'd like to get feedback before continuing.

1. Is the approach via an option what you want to go for? Or do you consider reverting the style change for 2024 (as @JelleZijlstra hinted in Allow a single newline after opening blocks, possibly via an option setting #3551 (comment))?
2. What about the name of the option? I went for allow_block_newline but am of course fine to change. (For similar options you rather went with skip_*, so skip_removing_block_newline?)

(Another possibility would be to do block_newline = Literal["remove", "allow", "enforce"], just to throw that into the discussion.)

[JelleZijlstra]

I don't want a new option for this. We minimize the use of options so that Black's style is consistent. Whatever we end up with, it should be the same for all users.

[return42]

I don't want a new option for this.

@JelleZijlstra OK / I understand ... but ...

We minimize the use of options so that Black's style is consistent. Whatever we end up with, it should be the same for all users.

when pin Black is not a alternative and an option is not wanted .. what alternatives are you suggest?

EDIT: haven't seen your comment #3551 (comment)

[JelleZijlstra]

when pin Black is not a alternative and an option is not wanted .. what alternatives are you suggest?

Either you pin Black, and always get the same formatting, or you don't pin it, and you sometimes get changes (but only when a new year comes around or you use --preview). You can't have it both ways.

[mathause]

Either you pin Black, and always get the same formatting, or you don't pin it, and you sometimes get changes (but only when a new year comes around or you use --preview). You can't have it both ways.

I don't think the problem is that applying black leads to changes but with this specific change that makes the code less readable (at least that is my issue).