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.

首先定位内置 setattr 的 typeshed 存根，并查看 python/typing#801 中的相关讨论。确定签名应如何拒绝 NoneType 和 EllipsisType，同时不会错误地排除其他有效目标；当预期的调用被报告为类型错误，并且现有的 setattr 用例仍受支持时，即表示完成。