Return a list of |_SerializedPart| instances corresponding to the
        parts in *phys_reader* accessible by walking the relationship graph
        starting with *pkg_srels*.

def _load_serialized_parts(phys_reader, pkg_srels, content_types):
        """
        Return a list of |_SerializedPart| instances corresponding to the
        parts in *phys_reader* accessible by walking the relationship graph
        starting with *pkg_srels*.
        """
        sparts = []
        part_walker = PackageReader._walk_phys_parts(phys_reader, pkg_srels)
        for partname, blob, srels in part_walker:
            content_type = content_types[partname]
            spart = _SerializedPart(partname, content_type, blob, srels)
            sparts.append(spart)
        return tuple(sparts)