[BEAM-3981] Cleanup of coders futurization.
[BEAM-3981] Cleanup of coders futurization.

Specifying language_level=3 allows the dict.items() optimization.

This is about a 40% speedup for 1-element dicts as well as cleaner code.

lukecwik/incubator-beam,robertwb/incubator-beam,apache/beam,robertwb/incubator-beam,chamikaramj/beam,RyanSkraba/beam,chamikaramj/beam,charlesccychen/beam,lukecwik/incubator-beam,markflyhigh/incubator-beam,chamikaramj/beam,iemejia/incubator-beam,lukecwik/incubator-beam,charlesccychen/incubator-beam,lukecwik/incubator-beam,RyanSkraba/beam,chamikaramj/beam,markflyhigh/incubator-beam,lukecwik/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,chamikaramj/beam,RyanSkraba/beam,mxm/incubator-beam,RyanSkraba/beam,apache/beam,chamikaramj/beam,rangadi/beam,charlesccychen/beam,apache/beam,mxm/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,apache/beam,rangadi/incubator-beam,rangadi/beam,rangadi/incubator-beam,charlesccychen/beam,markflyhigh/incubator-beam,charlesccychen/beam,charlesccychen/beam,RyanSkraba/beam,markflyhigh/incubator-beam,charlesccychen/beam,robertwb/incubator-beam,charlesccychen/incubator-beam,charlesccychen/beam,markflyhigh/incubator-beam,robertwb/incubator-beam,apache/beam,robertwb/incubator-beam,rangadi/beam,rangadi/beam,robertwb/incubator-beam,apache/beam,apache/beam,robertwb/incubator-beam,iemejia/incubator-beam,rangadi/beam,apache/beam,apache/beam,charlesccychen/incubator-beam,apache/beam,lukecwik/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,markflyhigh/incubator-beam,RyanSkraba/beam,rangadi/beam,RyanSkraba/beam,markflyhigh/incubator-beam,lukecwik/incubator-beam,rangadi/incubator-beam,lukecwik/incubator-beam,apache/beam,robertwb/incubator-beam,rangadi/beam,chamikaramj/beam