bpo-32528: Make asyncio.CancelledError a BaseException. (GH-13528)

This will address the common mistake many asyncio users make:
an "except Exception" clause breaking Tasks cancellation.

In addition to this change, we stop inheriting asyncio.TimeoutError
and asyncio.InvalidStateError from their concurrent.futures.*
counterparts.  There's no point for these exceptions to share the
inheritance chain.

In 3.9 we'll focus on implementing supervisors and cancel scopes,
which should allow better handling of all exceptions, including
SystemExit and KeyboardInterrupt

Lib/test/test_asyncio/test_tasks.py

@@ -1527,7 +1527,7 @@ class BaseTaskTests:
         async def sleeper():
             await asyncio.sleep(10)
 
-        base_exc = BaseException()
+        base_exc = SystemExit()
 
         async def notmutch():
             try:
@@ -1541,7 +1541,7 @@ class BaseTaskTests:
         task.cancel()
         self.assertFalse(task.done())
 
-        self.assertRaises(BaseException, test_utils.run_briefly, loop)
+        self.assertRaises(SystemExit, test_utils.run_briefly, loop)
 
         self.assertTrue(task.done())
         self.assertFalse(task.cancelled())