[Twisted-Python] new code: 3.5 or 3.6?

[Ian Haywood]

I've followed the discussion re 3.6 and type annotations, but evidently 
not closely enough

when writing new code for twisted today, can we use 3.6 features or is 
3.5 still required? (it's actually a library feature enum.IntFlag I'm 
curious about, not type annotations)

Ian