As expected, the workaround adding a delay of 500 ms is not perfect, the test still fails on the very slow x86 Windows7 3.6 buildbot when tests are run in parallel:

http://buildbot.python.org/all/#/builders/90/builds/383

======================================================================
FAIL: test_stdin_broken_pipe (test.test_asyncio.test_subprocess.SubprocessProactorTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\cygwin\home\db3l\buildarea\3.6.bolen-windows7\build\lib\test\test_asyncio\test_subprocess.py", line 221, in test_stdin_broken_pipe
    self.loop.run_until_complete, coro)
AssertionError: (<class 'BrokenPipeError'>, <class 'ConnectionResetError'>) not raised by run_until_complete


But hopefully, test_asyncio passed when re-run sequentially.