code: def iter_srels(self):
          
          for srel in self._pkg_srels:
              yield (PACKAGE_URI, srel)
          for spart in self._sparts:
              for srel in spart.srels:
                  yield (spart.partname, srel)
docs: Generate a 2-tuple `(source_uri, srel)` for each of the relationships
          in the package.