Convert sets to lists in doctests for Python 3.
Convert sets to lists in doctests for Python 3.

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