I've seen some stubs like
```
class X:
def __init__(self, x: str) -> None:
self.x = x
self.y = 0
```
I think this should be written instead as
```
class X:
x: str
y: int
def __init__(self, x: str) -> None: ...
```
1. Mentioned review requirement for core contributors.
2. Made stub coding style on par with requirements specified on
the Mypy wiki (we can merge them now).
