ABC subclasses may break `issubclass(T, T)` invariant

Bug report

Bug description:

This was a nasty bug to track down. I really don't have a clue why this would happen. It seems to be triggered under these conditions:

• A root class (Root) inherits from ABC
• There's diamond-shaped inheritance for a sub-subclass (Multi)
• In Root.__init_subclass__, when Multi is initializing, a check is performed to see if First is a subclass of Second
• First is no longer a subclass of itself

from abc import ABC

class Root(ABC):
    def __init_subclass__(cls, **kwargs):
        if cls.__name__ == "Multi":
            # issubclass(Second, First)  # Wouldn't trigger the bug for Second
            issubclass(First, Second)  # Triggers the bug for First
            issubclass(Second, First)  # Doesn't trigger the bug for Second

class First(Root): ...
class Second(Root): ...
class Multi(First, Second): ...

assert not issubclass(First, First)  # This is the bug.
assert issubclass(Second, Second)

print("bug!")

CPython versions tested on:

3.12

Operating systems tested on:

macOS

提供された Python 再現コードを CPython 3.12 で実行し、Multi の初期化中に ABC.init_subclass と issubclass を追跡します。First がもはや issubclass(First, First) を満たさない理由を調査します。diamond 階層に対する不変条件のアサーションが通り、このケースの回帰テストカバレッジが追加されれば完了です。