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
```

discord/context_managers.py

@@ -30,7 +30,7 @@ def _typing_done_callback(fut):
     # just retrieve any exception and call it a day
     try:
         fut.exception()
-    except Exception:
+    except (asyncio.CancelledError, Exception):
         pass
 
 class Typing: