Default values for generic attributes are unsafe

Currently mypy accepts this unsafe code:

from typing import Generic, TypeVar, List
  
T = TypeVar('T')

class B(Generic[T]):
    x: List[T] = []

class C1(B[int]):
    pass

class C2(B[str]):
    pass

C1().x.append(42)
C2().x.append('Hm...')

A potential solution would be to disallow all default values for generic attributes except None. This is however a low priority, since it is a false negative, and a rare corner case.

Start by reproducing the issue with the Python example in the report and inspect mypy's handling of generic attributes and default values. Determine the intended diagnostic for the shared mutable default, then add regression coverage showing that generic specializations cannot unsafely share it and verify the existing example is rejected.