Allow to use function objects as singletons

I was trying to create some stubs for lxml library, but I encountered a function that depending on a function object it gets (as in specific function object - checked with identity check) only finds objects of specific type.

Here's a very simplified mock-up of it so that you can get some idea on what I mean (the typing for arg argument in the find function is incorrect and that's what I want to be able to type hint in some way):

from typing import Union

class _Element: ...

def Element() -> _Element: ...

class _Comment: ...

def Comment() -> _Comment: ...

def find(arg: Union[Element, Comment]) -> Union[_Element, _Comment]:
    if arg is Element:
        # only find _Element objects
    elif arg is Element:
        # only find _Comment objects

# this function should only accept `Element` and `Comment` function object 
find(arg=Element)

So basically those functions serve as a singletons. Unfortunately typing.Union only allows Enums to work as singletons.

I am not entirely sure if this should be supported, usually the code could just be adjusted to use Enums, but in this case it will be used for type stubs so the code can't be edited. And to be honest, I am not sure if the code really needs editing in that case as if I didn't need typing for that library, I would say it's actually quite intuitive to use these factory functions as arguments in this case.

从 issue #668 中的 factory-function singleton 示例以及与 Enum singleton 的比较开始。由于 issue 中没有指定实现文件或测试，因此首先确定 typing 在哪里表示 singleton 值，以及函数对象目前是如何处理的。完成标准是确认这些函数对象是否可以在类型提示中被接受并进行区分表示，并覆盖 Element/Comment 风格的情况。