`isinstance` removes `cls.__dict__["__annotations__"]` in a complex setup with Protocol and ABCMeta

Bug report

Bug description:

import abc
from collections.abc import Iterator
from typing import Protocol

# ---- Expected behavior:
class Steppable(metaclass=abc.ABCMeta):
  pass

class Module:
  @classmethod
  def __init_subclass__(cls, **kwargs) -> None:
    super().__init_subclass__(**kwargs)
    name, annotation, default = "name", str | None, "abc"
    cls.__annotations__[name] = annotation
    setattr(cls, name, default)


class SequenceLayer1(Module, Steppable):
  pass


print(SequenceLayer1.__dict__.get("__annotations__"))
# {'name': str | None}
assert SequenceLayer1.__dict__.get("__annotations__") is not None


# ---- Unexpected behavior:
class CheckpointableIterator(Iterator, Protocol):
  pass

isinstance(CheckpointableIterator, Iterator)

class SequenceLayer2(Module, Steppable):
  pass

print(SequenceLayer2.__dict__.get("__annotations__"))
# None
assert SequenceLayer2.__dict__.get("__annotations__") is not None

• Failing on 3.12 and 3.13
• Passing on 3.11 and 3.14 (For 3.14 it is expected to have __dict__["__annotations__"] missing)

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Ejecuta el reproductor que involucra Module.init_subclass, CheckpointableIterator, Protocol, ABCMeta e isinstance en Python 3.12 y 3.13, y compáralo después con 3.11 y 3.14. Traza cómo SequenceLayer1 y SequenceLayer2 reciben annotations e identifica dónde la segunda clase pierde su entrada. Se considera terminado cuando el comportamiento está explicado y una prueba de regresión cubre el rango de versiones afectado.