Allow `multiprocessing` Servers to return non-callables

Feature or enhancement

Proposal:

The logic for the multiprocessing Server class is the following for retrieving values from an instance referenced by a proxy:

https://github.com/python/cpython/blob/7d369d471cf2b067c4d795d70b75201c48b46f5b/Lib/multiprocessing/managers.py#L273-L286

It works well the majority of the time, but it fails when trying to get something that doesn't immediately return a callable (it can probably be a bit more specific when it comes to what type of callable it is). A change like the following would yield more robust behavior from a base point of view and wouldn't require reimplementation in a subclass:

value_or_function = getattr(obj, methodname, SENTINEL) 

if value_or_function is SENTINEL:
    raise AttributeError(f"Type '{obj.__class__.__name__}' has no attribute named '{methodname}'")

if isinstance(value_of_function, typing.Callable):  
    try: 
        res = value_or_function(*args, **kwds) 
    except Exception as e: 
        msg = ('#ERROR', e) 
    else: 
        typeid = gettypeid and gettypeid.get(methodname, None) 
        if typeid: 
            rident, rexposed = self.create(conn, typeid, res) 
            token = Token(typeid, self.address, rident) 
            msg = ('#PROXY', (rexposed, token)) 
        else: 
            msg = ('#RETURN', res)
else:
    msg = ('#RETURN', value_or_function)

There are likely edge cases that the above does not account for, but a change similar to the one above will open up the use of instance member variables and getters for properties.

A value for SENTINEL (or better) would obviously have to be defined somewhere (it's not in the example).

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

从 Lib/multiprocessing/managers.py 中 issue 所链接的 Server 获取逻辑开始。跟踪 proxy 属性访问如何区分可调用值和返回值，包括属性缺失的情况以及 proxy 创建路径。完成标准是：无需重新实现即可获取实例成员变量和属性 getter，同时保持现有的可调用值行为不变。