Adopt Py3.8+ features: CancelledError is a subclass of BaseException

[Bibo-Joshi]

Python 3.8 makes asyncio.CancelledError as subclass of BaseException. As a consequence, it is no longer catched by except Exception clauses and we no longer need special casing in the style of except asyncio.CancelledError.
We review our code base for explicit usages of asyncio.CancelledError and check which ones of them we can drop.

A PR in this direction is very welcome. As long as #3728 is open, it should be based on the drop-py-37 branch. If you would like to send a PR, please be sure to read our contribution guide and leave a short comment so that we can assign you.