bpo-40275: Avoid importing socket in test.support (GH-19603)

* Move socket related functions from test.support to socket_helper.
* Import socket, nntplib and urllib.error lazily in transient_internet().
* Remove importing multiprocess.
diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 74aef47..362ba9e 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -30,6 +30,7 @@
 import uuid
 import warnings
 from test import support
+from test.support import socket_helper
 from platform import win32_is_iot
 
 try:
@@ -3171,7 +3172,7 @@
         support.unlink(support.TESTFN)
 
     def setUp(self):
-        self.server = SendfileTestServer((support.HOST, 0))
+        self.server = SendfileTestServer((socket_helper.HOST, 0))
         self.server.start()
         self.client = socket.socket()
         self.client.connect((self.server.host, self.server.port))