Remove unnecessary calls to iter
Remove unnecessary calls to iter

On Py3, dict.items, et. al return iterators. Calling iter
over these methods only served to slow things down for
marshmallow's use cases

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