bpo-32751: Wait for task cancellation in asyncio.wait_for() (GH-7216)

Currently, asyncio.wait_for(fut), upon reaching the timeout deadline,
cancels the future and returns immediately.  This is problematic for
when *fut* is a Task, because it will be left running for an arbitrary
amount of time.  This behavior is iself surprising and may lead to
related bugs such as the one described in bpo-33638:

    condition = asyncio.Condition()
    async with condition:
        await asyncio.wait_for(condition.wait(), timeout=0.5)

Currently, instead of raising a TimeoutError, the above code will fail
with `RuntimeError: cannot wait on un-acquired lock`, because
`__aexit__` is reached _before_ `condition.wait()` finishes its
cancellation and re-acquires the condition lock.

To resolve this, make `wait_for` await for the task cancellation.
The tradeoff here is that the `timeout` promise may be broken if the
task decides to handle its cancellation in a slow way.  This represents
a behavior change and should probably not be back-patched to 3.6 and
earlier.

Lib/test/test_asyncio/test_locks.py

@@ -807,6 +807,19 @@ class ConditionTests(test_utils.TestCase):
         with self.assertRaises(ValueError):
             asyncio.Condition(lock, loop=loop)
 
+    def test_timeout_in_block(self):
+        loop = asyncio.new_event_loop()
+        self.addCleanup(loop.close)
+
+        async def task_timeout():
+            condition = asyncio.Condition(loop=loop)
+            async with condition:
+                with self.assertRaises(asyncio.TimeoutError):
+                    await asyncio.wait_for(condition.wait(), timeout=0.5,
+                                           loop=loop)
+
+        loop.run_until_complete(task_timeout())
+
 
 class SemaphoreTests(test_utils.TestCase):