In the specific case of contextlib, most of the APIs should be able to transparently support async/await without negatively impacting their synchronous behaviour, so after the python-dev discussion, I think one module with separate sync and async test suites is a good way to go for contextlib specifically: https://mail.python.org/pipermail/python-dev/2017-March/147520.html

However, as noted in that message, I *don't* think we can conclude that's going to be the right answer for the standard library in general - for many modules, a separate a<module> or aio<module> published via PyPI and potentially considered for stdlib inclusion later is going to make more sense.