Can't peek at paused bytes (socket behind TCPTransport unavailable)

• uvloop version: 0.14.0
• Python version: 3.8.2
• Platform: linux
• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?: n/a (this is an api thing)
• Does uvloop behave differently from vanilla asyncio? How?:
  • vanilla asyncio: the underlying socket is available in transport._sock for consenting coders aware of the risks using "private" members
  • uvloop: the underlying socket is not available to python at all (or at least I can't find it via introspection)

FWIW, one reason I'm running into this right now is that I have a case where the transport is paused for reading (transport.pause_reading()), but I would still like to peek at the bytes with sock.recv(n, socket.MSG_PEEK).

首先比較 uvloop 的 TCP transport 與 vanilla asyncio 的 transport._sock 行為，重點關注讀取暫停時的存取。只有當回報的 MSG_PEEK 使用情境能夠透過 uvloop 以受支援的方式檢查暫停中的 socket 位元組，並且已根據 issue 的重現細節驗證行為時，才算完成。