As another datapoint, this also broke some of my code on 3.8 because I was using `concurrent.futures.CancelledError` rather than `asyncio.CancelledError` to handle cancelled futures. And I'm certainly not the only one to have done this given that it's mentioned in at least two Stack Overflow answers: https://stackoverflow.com/a/38655063 and https://stackoverflow.com/a/36277556

While I understand the rationale behind this change, it would've been good to include this inheritance detail in the 3.8 release notes.