Ability to weakly reference an instance of a class lost due to mutliple inheritance

This is related to https://github.com/python/cpython/issues/95589 due to underlying mechanism, but the behavior is distinct.

class C: 
    pass

class D(int, C):
    pass
>>> c = C()
>>> weakref.ref(c)
<weakref at 0x7fa5b2af9c60; to 'C' at 0x7fa5b2e501c0>
>>> d = D()
>>> weakref.ref(d)
TypeError: cannot create weak reference to 'D' object
>>> isinstance(d, C)
True

If I can weakly reference an instance of C, then I should be able to weakly reference an instance of D. Instances of D are instances of C

首先使用类 C 和 D 运行 issue 的 Python 复现程序，检查每个实例的 weakref.ref 行为。跟踪 CPython 对涉及多重继承的实例进行弱引用处理的过程；当 D 实例能够以与其 C 基类一致的方式进行弱引用时，即视为完成。