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_base_events.py

@@ -476,7 +476,7 @@ class BaseEventLoopTests(test_utils.TestCase):
             other_loop.run_until_complete, task)
 
     def test_run_until_complete_loop_orphan_future_close_loop(self):
-        class ShowStopper(BaseException):
+        class ShowStopper(SystemExit):
             pass
 
         async def foo(delay):
@@ -487,10 +487,8 @@ class BaseEventLoopTests(test_utils.TestCase):
 
         self.loop._process_events = mock.Mock()
         self.loop.call_soon(throw)
-        try:
+        with self.assertRaises(ShowStopper):
             self.loop.run_until_complete(foo(0.1))
-        except ShowStopper:
-            pass
 
         # This call fails if run_until_complete does not clean up
         # done-callback for the previous future.

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())