Added build matrix for multiple python versions.
Added build matrix for multiple python versions.

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