>This code could be made much safer if there were a PyType_FromSpecWithBasesMeta which used the meta class to allocate the memory

IMHO, `PyType_FromSpecWithBases` is a atomic API of `TypeObject`.
Adding a class as a param will result in `PyType_FromSpecWithBases` will be complicated(the logic of this function depends on the children's tp_alloc sometimes).

I add pter in this bpo, maybe he have some better ideas :)