Changed to use six.string_types.
Changed to use six.string_types.

cmshobe/landlab,laijingtao/landlab,amandersillinois/landlab,landlab/landlab,Carralex/landlab,landlab/landlab,csherwood-usgs/landlab,ManuSchmi88/landlab,decvalts/landlab,RondaStrauch/landlab,decvalts/landlab,Carralex/landlab,ManuSchmi88/landlab,Carralex/landlab,csherwood-usgs/landlab,RondaStrauch/landlab,landlab/landlab,cmshobe/landlab,ManuSchmi88/landlab,RondaStrauch/landlab,cmshobe/landlab,amandersillinois/landlab,SiccarPoint/landlab,SiccarPoint/landlab,laijingtao/landlab