          if isinstance(source_srs, SpatialReference):
              sr = source_srs
          elif isinstance(source_srs, self.spatial_backend.spatial_ref_sys()):
              sr = source_srs.srs
          elif isinstance(source_srs, (int, six.string_types)):
              sr = SpatialReference(source_srs)
          else:
              # Otherwise just pulling the SpatialReference from the layer
              sr = self.layer.srs
  
 
</s> remove         elif isinstance(srs, six.integer_types + six.string_types):
 </s> add         elif isinstance(srs, (int, str)): </s> remove         elif isinstance(coord_trans, six.integer_types + six.string_types):
 </s> add         elif isinstance(coord_trans, (int, str)): </s> remove         elif isinstance(value, six.integer_types + six.string_types):
 </s> add         elif isinstance(value, (int, str)): </s> remove         elif isinstance(wkb, (bytes, six.string_types)):
 </s> add         elif isinstance(wkb, (bytes, str)): </s> remove         elif f.geom_type == 'RASTER' and isinstance(value, six.string_types):
 </s> add         elif f.geom_type == 'RASTER' and isinstance(value, str):
        elif isinstance(source_srs, (int, str)):
          if isinstance(source_srs, SpatialReference):
              sr = source_srs
          elif isinstance(source_srs, self.spatial_backend.spatial_ref_sys()):
              sr = source_srs.srs
        elif isinstance(source_srs, (int, str)):
              sr = SpatialReference(source_srs)
          else:
              # Otherwise just pulling the SpatialReference from the layer
              sr = self.layer.srs
  
keep keep keep keep replace keep keep keep keep keep
 Refs #23919 -- Removed six.<various>_types usage

Thanks Tim Graham and Simon Charette for the reviews. 
django/contrib/gis/utils/layermapping.py