Return `limit` number of ancestor blocks from the current canonical head.

def get_ancestors(self, limit: int, header: BlockHeader) -> Tuple[BaseBlock, ...]:
        """
        Return `limit` number of ancestor blocks from the current canonical head.
        """
        ancestor_count = min(header.block_number, limit)

        # We construct a temporary block object
        vm_class = self.get_vm_class_for_block_number(header.block_number)
        block_class = vm_class.get_block_class()
        block = block_class(header=header, uncles=[])

        ancestor_generator = iterate(compose(
            self.get_block_by_hash,
            operator.attrgetter('parent_hash'),
            operator.attrgetter('header'),
        ), block)
        # we peel off the first element from the iterator which will be the
        # temporary block object we constructed.
        next(ancestor_generator)

        return tuple(take(ancestor_count, ancestor_generator))