Improving the type signature of `setattr`

When the first argument of setattr is of type NoneType or EllipsisType the function will always raise an AttributeError on runtime, so it would be nice if the following calls would be detected as type errors.

x = None
setattr(x, 'foo', True) # AttributeError: 'NoneType' object has no attribute 'test'
y = ...
setattr(y, 'foo', True) # AttributeError: 'ellipsis' object has no attribute 'test'

I am not sure what's the best way to go about this is, there are probably many builtin types that do not support arbitrary attributes to be set. We do not appear to have a type to categorize such types, which could then in the future be used with Not[], once python/typing#801 is implemented.

まず、組み込みの setattr の typeshed スタブを見つけ、python/typing#801 の関連する議論を確認します。ほかの有効な対象を誤って除外せずに、シグネチャが NoneType と EllipsisType を拒否する方法を決定します。意図した呼び出しが型エラーとして報告され、既存の setattr のケースが引き続きサポートされれば完了です。