It seems test_asyncio fails sporadically on AppVeyor:
https://ci.appveyor.com/project/python/cpython/build/3.7.0a0.10081#L2650

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ERROR: test_start_tls_server_1 (test.test_asyncio.test_sslproto.ProactorStartTLS)
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Traceback (most recent call last):
  File "C:\projects\cpython\lib\test\test_asyncio\test_sslproto.py", line 284, in test_start_tls_server_1
    asyncio.wait_for(main(), loop=self.loop, timeout=10))
  File "C:\projects\cpython\lib\asyncio\base_events.py", line 440, in run_until_complete
    return future.result()
  File "C:\projects\cpython\lib\asyncio\tasks.py", line 398, in wait_for
    raise futures.TimeoutError()
concurrent.futures._base.TimeoutError
======================================================================
FAIL: test_start_tls_server_1 (test.test_asyncio.test_sslproto.ProactorStartTLS)
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Traceback (most recent call last):
  File "C:\projects\cpython\lib\test\test_asyncio\functional.py", line 43, in tearDown
    self.fail('unexpected calls to loop.call_exception_handler()')
AssertionError: unexpected calls to loop.call_exception_handler()