Avoid introducing new parentheses in annotated assignments (#8233)
## Summary
We decided to avoid changing this in
https://github.com/astral-sh/ruff/issues/7315, but it's been reported
multiple times (e.g., in https://github.com/astral-sh/ruff/issues/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 |
