Fix return value of dumps on Py3
Fix return value of dumps on Py3

quxiaolong1504/marshmallow,bartaelterman/marshmallow,dwieeb/marshmallow,daniloakamine/marshmallow,jmcarp/marshmallow,Bachmann1234/marshmallow,0xDCA/marshmallow,0xDCA/marshmallow,VladimirPal/marshmallow,marshmallow-code/marshmallow,etataurov/marshmallow,jmcarp/marshmallow,xLegoz/marshmallow,Tim-Erwin/marshmallow,maximkulkin/marshmallow,mwstobo/marshmallow