`isinstance` on multiple-inheritance class erases type hints

Bug report

Bug description:

Steps to reproduce:

• Create a runtime checkable protocol
• Create one base class with type hints
• Create a second base class that satisfies the protocol
• Create a derived class that inherits from both the base classes
• get_type_hints on the derived class and see they are correct
• check the derived class isinstance of the protocol
• get_type_hints on the derived class and see that they are a blank dictionary

from abc import abstractmethod
from typing import Protocol, get_type_hints, runtime_checkable

@runtime_checkable
class Hello(Protocol):
    @abstractmethod
    def hello(self) -> str: ...

class Base:
    a: str

class Base2:
    def hello(self) -> str:
        return "hello"

class Thing(Base, Base2): ...

def test_hints():
    t = Thing()
    assert get_type_hints(t) == {"a": str}
    assert isinstance(t, Hello)
    assert get_type_hints(t) == {"a": str}

CPython versions tested on:

3.11, 3.13

Operating systems tested on:

Linux

首先在 CPython 上运行提供的复现程序，并比较 isinstance 检查前后的 get_type_hints。跟踪该检查涉及的 get_type_hints 和 runtime-checkable Protocol 路径。完成的标准是：在 isinstance(t, Hello) 之后，派生的 Thing 实例仍报告 {"a": str}。