Transport sockets do not support shutdown() method

• uvloop version: 0.12.2
• Python version: 3.5.3
• Platform: Debian 9
• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?: yes

An attempt to use shutdown method of transport socket causes following exception:

source_traceback: Object created at (most recent call last):
  File "/usr/local/bin/ssh-tarpit", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.5/dist-packages/ssh_tarpit/__main__.py", line 109, in main
    loop.run_until_complete(amain(args, loop))
  File "/usr/lib/python3.5/asyncio/streams.py", line 238, in connection_made
    self._stream_writer)
  File "/usr/local/lib/python3.5/dist-packages/ssh_tarpit/server.py", line 52, in _spawn
    self._loop.create_task(self.handler(reader, writer)))
Traceback (most recent call last):
  File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
    result = coro.send(None)
  File "/usr/lib/python3.5/asyncio/coroutines.py", line 125, in send
    return self.gen.send(value)
  File "/usr/local/lib/python3.5/dist-packages/ssh_tarpit/server.py", line 38, in handler
    sock.shutdown(socket.SHUT_RD)
  File "uvloop/pseudosock.pyx", line 155, in uvloop.loop.PseudoSocket.shutdown
  File "uvloop/pseudosock.pyx", line 19, in uvloop.loop.PseudoSocket._na
TypeError: transport sockets do not support shutdown() method

Such behavior is not observed with default event loop.

阅读 uvloop/pseudosock.pyx 中的 PseudoSocket.shutdown 以及 traceback 中显示的 _na 路径；首先使用报告中的 Python 和 uvloop 版本重现 socket shutdown 调用。完成标准是 transport socket 能够处理 shutdown 而不会出现报告的 TypeError，并且行为与默认 event loop 一致。