message: Change _DictFetchMapper to not use __setitem__ on a Mapping, since these may be immutable.
diff: @@ -413,10 +413,9 @@ class _DictFetchMapper(_FetchMapper):
  return self._unique_fetches
  def build_results(self, values):
  - results = self._fetch_type()
  - for k, m, vi in zip(self._keys, self._mappers, self._value_indices):
  - results[k] = m.build_results([values[j] for j in vi])
  - return results
  + return self._fetch_type(
  + (k, m.build_results([values[j] for j in vi]))
  + for k, m, vi in zip(self._keys, self._mappers, self._value_indices))
  class _AttrsFetchMapper(_FetchMapper):
  