asyncio, Tulip issue 137: In debug mode, save traceback where Future, Task and

Handle objects are created. Pass the traceback to call_exception_handler() in
the 'source_traceback' key.

The traceback is truncated to hide internal calls in asyncio, show only the
traceback from user code.

Add tests for the new source_traceback, and a test for the 'Future/Task
exception was never retrieved' log.

Lib/test/test_asyncio/test_base_events.py

@@ -406,19 +406,22 @@ class BaseEventLoopTests(test_utils.TestCase):
             1/0
 
         def run_loop():
-            self.loop.call_soon(zero_error)
+            handle = self.loop.call_soon(zero_error)
             self.loop._run_once()
+            return handle
 
+        self.loop.set_debug(True)
         self.loop._process_events = mock.Mock()
 
         mock_handler = mock.Mock()
         self.loop.set_exception_handler(mock_handler)
-        run_loop()
+        handle = run_loop()
         mock_handler.assert_called_with(self.loop, {
             'exception': MOCK_ANY,
             'message': test_utils.MockPattern(
                                 'Exception in callback.*zero_error'),
-            'handle': MOCK_ANY,
+            'handle': handle,
+            'source_traceback': handle._source_traceback,
         })
         mock_handler.reset_mock()