Remove set() casts in 0.5

[chrislit]

The following line in Millar() & Clark() can be simplified:
alphabet = set(set(src_tok.keys()) | set(tar_tok.keys()))

to:
alphabet = set(src_tok.keys() | tar_tok.keys())

as part of removing Python 2.7 support

[chrislit]

done at 665be22