Thank you for adding these tests! I just have one suggestion to make the test closer to how these methods will be used in typical code. The idea is that there will be typically a pair of classes:

class Generic:
    def __class_getitem__(cls, item):
        return GenericAlias(item)

class GenericAlias:
    def __init__(self, item):
        self.item = item
    def __mro_entries__(self, bases):
        return (item,)

but this is just a minor suggestion, you can merge it as is if you prefer.