Avoid introducing new parentheses in annotated assignments

[charliermarsh]

Summary

We decided to avoid changing this in #7315, but it's been reported multiple times (e.g., in #8226, also on Discord). I suggest we change it to improve compatibility. In general, it also seems to lend itself to better code style.

Closes #8188
Closes #8226

Test Plan

Shows improvements for CPython, home-assistant, Poetry, and typeshed.

Before:

project	similarity index	total files	changed files
cpython	0.75803	1799	1647
django	0.99983	2772	34
home-assistant	0.99953	10596	186
poetry	0.99891	317	17
transformers	0.99966	2657	330
twine	1.00000	33	0
typeshed	0.99978	3669	20
warehouse	0.99977	654	13
zulip	0.99970	1459	22

After:

project	similarity index	total files	changed files
cpython	0.75804	1799	1647
django	0.99983	2772	34
home-assistant	0.99960	10596	156
poetry	0.99897	317	17
transformers	0.99966	2657	330
twine	1.00000	33	0
typeshed	0.99980	3669	18
warehouse	0.99977	654	13
zulip	0.99970	1459	22

[MichaReiser]

We should update the documentation too.

Are there type annotations in other positions that need updating too?

I'm somewhat torn on this (see comment in #8188) because I think the unfortunate formatting will be fixed when implementing the preview style. Also, there are now cases (with unions on the left), that now exceed the line width, but didn't before. Should we special case instead to only avoid parenthesizing if the type annotation is a single name?

Could we gate the old behavior behind the preview flag? I think the old formatting yields better results if we parenthesize the left, if parenthesizing the right didn't make it fit.

[MichaReiser]

Suggested change

	[target.format(), token(":"), space(), annotation.format(),]
	[target.format(), token(":"), space(), annotation.format()]

[charliermarsh]

Went ahead and updated the docs. My current thinking here is that we should change this based on improving Black compatibility. The current formatting is better in some cases, e.g., from the docs:

# Black
StartElementHandler: Callable[[str, dict[str, str]], Any] | Callable[[str, list[str]], Any] | Callable[
    [str, dict[str, str], list[str]], Any
] | None

# Ruff
StartElementHandler: (
    Callable[[str, dict[str, str]], Any]
    | Callable[[str, list[str]], Any]
    | Callable[[str, dict[str, str], list[str]], Any]
    | None
)

But I think we can improve those by making progress on preview style -- and complex union annotations are also much rarer than simple annotations.