docs: def _write_parts(phys_writer, parts):
          """
          Write the blob of each part in *parts* to the package, along with a
          rels item for its relationships if and only if it has any.
          """
          for part in parts:
              phys_writer.write(part.partname, part.blob)
              if len(part._rels):
                  phys_writer.write(part.partname.rels_uri, part._rels.xml)