test_asyncgen.test_async_gen_asyncio_gc_aclose_09() fails randomly. It looks like a race condition, likely related to the system load.

https://ci.appveyor.com/project/python/cpython/build/3.6build10993/job/l89045et4rmey72q

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Run tests in parallel using 4 child processes
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test_async_gen_asyncio_gc_aclose_09 (test.test_asyncgen.AsyncGenAsyncioTest) ... Task was destroyed but it is pending!
task: <Task pending coro=<async_generator_athrow()> wait_for=<Future finished result=None>>
FAIL

======================================================================
FAIL: test_async_gen_asyncio_gc_aclose_09 (test.test_asyncgen.AsyncGenAsyncioTest)
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Traceback (most recent call last):
  File "C:\projects\cpython\lib\test\test_asyncgen.py", line 630, in test_async_gen_asyncio_gc_aclose_09
    self.assertEqual(DONE, 1)
AssertionError: 0 != 1
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Re-running test 'test_asyncgen' in verbose mode
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test_async_gen_asyncio_gc_aclose_09 (test.test_asyncgen.AsyncGenAsyncioTest) ... ok
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Ran 40 tests in 1.125s