Put blank lines between nested classes in typing stubs

[MarkKoz]

Describe the style change

Nested classes in typing stubs should have blank lines between them.

Examples in the current Black style

class A:
    class B:
        attribute: int
    class C:
        attribute: int
    def method(self) -> None: ...
    def method2(self) -> None: ...

Desired style

class A:
    class B:
        attribute: int
    
    class C:
        attribute: int
    
    def method(self) -> None: ...
    def method2(self) -> None: ...

Additional context

See this for a live demo.

[JelleZijlstra]

I'm in favor of making this change, but cc fellow typeshed maintainers @srittau @Akuli @hauntsaninja.

[hauntsaninja]

Yeah, seems nice.

Somewhat related, the stubs style change I want from black is for top-level code under an if-statement to have the same blank lines as top-level code proper. Example here

[JelleZijlstra]

@hauntsaninja we made some changes around that in #2472, but agree that we should space in your example too. Feel free to open a new issue. We also have an unreleased change in #2736.

[Akuli]

I'm fine with this change.

[JelleZijlstra]

Everyone agrees, so @MarkKoz feel free to go ahead and implement this. Thanks!

[jpy-git]

Had a quick look at this and, since there's no special consideration given to whether an indented block is within a class or an if, it seems the work in #2820 and #2796 has partially addressed this issue.
There are however there are a couple other cases that need to be accounted for in both if and class blocks: if example and class example

For stubs, it feels like making any indented block consistent with the top level achieves the goal of this issue and #2784.

Although one place I can see class and if differing is the gap between attributes and methods (example) since this may introduce a lot of changes in typeshed.