Something that occurred to me as being a bit tricky to handle here is the backport to contextlib2: that maintains compatibility with 2.6+, so it would need to split any code using "async def" and "await" out to a separate file that only gets imported on 3.5+ (and similarly only run the corresponding test cases on 3.5+).

A potentially simpler alternative to that would be to create a new "backports.contextlib" package that only supports 3.5+ and explicitly restrict contextlib2 itself to code that runs in the common subset of Python 2 & 3.