Restrict PYI034 for in-place operations to enclosing class (#24511)
## Summary
In `.py` and `.pyi` files, we now only flag cases in which the return
type is the enclosing class, like:
```python
class A:
def __iadd__(self) -> A:
return self
```
As opposed to:
```python
class A:
def __iadd__(self) -> int:
return self
```
Closes https://github.com/astral-sh/ruff/issues/24462.