Changed behavior of singledispatch in Python 3.12

Bug report

Bug description:

It seems the behavior of functools.singledispatch changed in Python 3.12 in situations where a typing.Protocol is involved.

When executing the following code on 3.12, MyClass no longer gets dispatched as via the default but as MyProtocol. The problem seems to be that issubclass now behaves differently when called on protocols. The issue was noticed here where it caused downstream problems.

from typing import Protocol
from functools import singledispatch


class MyProtocol(Protocol):
    pass

class MyClass:
    pass

s = singledispatch(lambda x: "default")
s.register(MyProtocol, lambda x: "protocol")

print(s(MyClass))  # yields "default" on 3.11 but "protocol" on 3.12

CPython versions tested on:

3.11, 3.12

Operating systems tested on:

macOS