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

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