DOC: fix README, CancelledError is a BaseException since 3.8

[cjrh]

The README currently says this:

Consider the previous example using aiohttp to fetch URLs: inside the fetch() function, we're handling Exception, which includes asyncio.CancelledError

This is incorrect since Python 3.8, when the superclass of asyncio.CancelledError changed from Exception to BaseException. See https://docs.python.org/3/library/asyncio-exceptions.html#asyncio.CancelledError