`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

まず、提供された再現コードを CPython で実行し、isinstance チェックの前後で get_type_hints を比較します。そのチェックに関係する get_type_hints と runtime-checkable Protocol のパスを追跡します。isinstance(t, Hello) の後も派生した Thing インスタンスが {"a": str} を報告すれば完了です。