asyncio: use directly socket.socketpair() (#4597)

Since Python 3.5, socket.socketpair() is also available on Windows,
and so can be used directly, rather than using
asyncio.windows_utils.socketpair().

Lib/test/test_asyncio/test_windows_events.py

@@ -1,4 +1,5 @@
 import os
+import socket
 import sys
 import unittest
 from unittest import mock
@@ -36,7 +37,7 @@ class ProactorTests(test_utils.TestCase):
         self.set_event_loop(self.loop)
 
     def test_close(self):
-        a, b = self.loop._socketpair()
+        a, b = socket.socketpair()
         trans = self.loop._make_socket_transport(a, asyncio.Protocol())
         f = asyncio.ensure_future(self.loop.sock_recv(b, 100))
         trans.close()