bpo-30048: asyncio: fix Task.cancel() was ignored. (GH-1547)

* bpo-30048: asyncio: fix Task.cancel() was ignored. (GH-1097)

when there are no more `await` or `yield (from)` before return in coroutine,
cancel was ignored.

example:

    async def coro():
        asyncio.Task.current_task().cancel()
        return 42
    ...
    res = await coro()  # should raise CancelledError
(cherry picked from commit 991adca012f5e106c2d4040ce619c696ba6f9c46)

* fix test

1 files changed, 3 insertions, 0 deletions

diff --git a/Misc/NEWS b/Misc/NEWS
index 889386d..41fe80f 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -49,6 +49,9 @@ Extension Modules
 Library
 -------
 
+- bpo-30048: Fixed ``Task.cancel()`` can be ignored when the task is
+  running coroutine and the coroutine returned without any more ``await``.
+
 - bpo-29990: Fix range checking in GB18030 decoder.  Original patch by Ma Lin.
 
 - Revert bpo-26293 for zipfile breakage. See also bpo-29094.