[dataclass_transform] fix frozen behavior for base classes with direct metaclasses

[wesleywright]

Fixes #14857. The initial implementation overlooked this statement in PEP 681:

Similarly, a class that directly specifies a metaclass that is decorated with dataclass_transform is considered neither frozen nor non-frozen.

As far as I can tell, this is a special case that only applies to classes that directly specify a dataclass_transform metaclass. This is import for projects like Pydantic, which requires clients to use a base class but still supports a frozen parameter.

Note that this allows mixed frozen and non-frozen behavior if the base class does have fields:

@dataclass_transform()
class Meta(type): ...
class Base(metaclass=Meta, frozen=False):
    base: int = 0
class Foo(Base, frozen=True):
    foo: int = 0

foo = Foo()
foo.foo = 1 # an error because Foo is frozen
foo.base = 1 # NOT an error — Base is neither frozen nor non-frozen

While this is probably surprising behavior, it seems to match the text of the PEP as well as the behavior of Pyright.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JukkaL]

Thanks, looks good!