STYLE Reduce merge conflicts with `force_single_line` or `force_grid_wrap=1`

[MarcoGorelli]

About 2 years ago, we introduced force_grid_wrap to reduce merge conflicts due to imports #39780

This helped, but we do better in the case when there's a single import per group.

For example, in #49736 the diff was:

- from typing import Iterator
+ from typing import (
+     Generic,
+     Iterator,
+     TypeVar,
+ )

, which caused conflicts when backporting to 1.5.x, because 1.5.x didn't have the Iterator import

If the import style had instead been:

from typing import Generic
from typing import Iterator
from typing import TypeVar

then the PR above could have just added the Generic and TypeVar imports, without touching the Iterator import, and the backport wouldn't have had this conflict

As an aside, this would also remove nearly 5,000 lines of code

---

This is how it would look like: #50275

---

An alternative would be to have forced

from typing import (
    Iterator,
)

, see #51693

This makes imports a lot longer, but has the same effect of reducing conflicts

[bashtage]

Thanks for knocking up the alternative.

I suppose both are a bit terrible but also both acceptable. The one full import per line is very natural with a single import, while the always using multiline is natural, at least for black users, with multiple imports. Ultimately I think both are worth the effort to improve the merge situation, so let's just let a majority of interested parties express an opinion and go with that.

[MarcoGorelli]

closing as in the end people weren't too keen on this