Return rels item XML for source with *source_uri* or None if no rels
        item is present.

def rels_xml_for(self, source_uri):
        """
        Return rels item XML for source with *source_uri* or None if no rels
        item is present.
        """
        try:
            rels_xml = self.blob_for(source_uri.rels_uri)
        except KeyError:
            rels_xml = None
        return rels_xml