ApplyTypeAnnotationsVisitor does not respect overwrite_existing_annotations for attributes

[shrik450]

Consider this example:

class X:
    x: int

used to patch:

class X:
    x: str

ApplyTypeAnnotationsVisitor will not change the type annotation on X.x to int even if you set overwrite_existing_annotations to True. This is the case even if you change it to be an assign (as in x: str = "a".)