test_signal.test_warn_on_full_buffer() failed on AppVeyor.

https://ci.appveyor.com/project/python/cpython/build/3.8build19372

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FAIL: test_warn_on_full_buffer (test.test_signal.WakeupSocketSignalTests)
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Traceback (most recent call last):
  File "C:\projects\cpython\lib\test\test_signal.py", line 538, in test_warn_on_full_buffer
    assert_python_ok('-c', code)
  File "C:\projects\cpython\lib\test\support\script_helper.py", line 157, in assert_python_ok
    return _assert_python(True, *args, **env_vars)
  File "C:\projects\cpython\lib\test\support\script_helper.py", line 143, in _assert_python
    res.fail(cmd_line)
  File "C:\projects\cpython\lib\test\support\script_helper.py", line 84, in fail
    err))
AssertionError: Process return code is 1
command line: ['C:\\projects\\cpython\\PCbuild\\win32\\python.exe', '-X', 'faulthandler', '-I', '-c', 'if 1:\n        import errno\n        import signal\n        import socket\n        import sys\n        import time\n        import _testcapi\n        from test.support import captured_stderr\n\n        signum = signal.SIGINT\n\n        # This handler will be called, but we intentionally won\'t read from\n        # the wakeup fd.\n        def handler(signum, frame):\n            pass\n\n        signal.signal(signum, handler)\n\n        read, write = socket.socketpair()\n        read.setblocking(False)\n        write.setblocking(False)\n\n        # Fill the send buffer\n        try:\n            while True:\n                write.send(b"x")\n        except BlockingIOError:\n            pass\n\n        # By default, we get a warning when a signal arrives\n        signal.set_wakeup_fd(write.fileno())\n\n        with captured_stderr() as err:\n            _testcapi.raise_signal(signum)\n\n        err = err.getvalue()\n        if (\'Exception ignored when trying to send to the signal wakeup fd\'\n            not in err):\n            raise AssertionError(err)\n\n        # And also if warn_on_full_buffer=True\n        signal.set_wakeup_fd(write.fileno(), warn_on_full_buffer=True)\n\n        with captured_stderr() as err:\n            _testcapi.raise_signal(signum)\n\n        err = err.getvalue()\n        if (\'Exception ignored when trying to send to the signal wakeup fd\'\n            not in err):\n            raise AssertionError(err)\n\n        # But not if warn_on_full_buffer=False\n        signal.set_wakeup_fd(write.fileno(), warn_on_full_buffer=False)\n\n        with captured_stderr() as err:\n            _testcapi.raise_signal(signum)\n\n        err = err.getvalue()\n        if err != "":\n            raise AssertionError("got unexpected output %r" % (err,))\n\n        # And then check the default again, to make sure warn_on_full_buffer\n        # settings don\'t leak across calls.\n        signal.set_wakeup_fd(write.fileno())\n\n        with captured_stderr() as err:\n            _testcapi.raise_signal(signum)\n\n        err = err.getvalue()\n        if (\'Exception ignored when trying to send to the signal wakeup fd\'\n            not in err):\n            raise AssertionError(err)\n\n        ']
stdout:
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stderr:
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Traceback (most recent call last):
  File "<string>", line 39, in <module>
AssertionError
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The test passed when run again sequentially ("Re-running test 'test_signal' in verbose mode").