Catch asyncio.CancelledError in 3.8 in typing context manager
Catch asyncio.CancelledError in 3.8 in typing context manager

In python 3.8, asyncio.CancelledError is a subclass of BaseException
rather than Exception, so `except Exception:` will not swallow
CancelledError. This change prevents an error in 3.8 from being printed
to the console when the following is run:

```
async with ctx.typing():
	pass
```

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