co_annotations fails with non dict return from __prepare__

[zrothberg]

Currently __prepare__ only requires that the returned item implements __setitem__ and __getitem__. It appears that co_annotations require that the object actually must be a subclass of dict. Minimum code to reproduce.

class TestDict:
    def __init__(self):
        self.data = {}
    def __setitem__(self, key, value):
        self.data[key] = value
    def __getitem__(self, item):
        return self.data[item]

class CustomType(type):
    @classmethod
    def __prepare__(metacls, name, bases):
        return TestDict()
    def __new__(metacls, name, bases, namespace, /, **kwargs):
        namespace = namespace.data
        cls = super().__new__(metacls, name, bases, namespace, **kwargs)
        return cls


class A(metaclass=CustomType):
    val:str

A.__annotations__
#errors here 

__new__ does require that the input to its third argument be a subclass of dictionary but any metaclass that handles it like above would be avoiding that issue.

Not sure if this is just a bad assertion and its not actually an issue for the code or if it is an issue with the code.