Issue #7449, last part (11): fix many tests if thread support is disabled

 * Use try/except ImportError or test_support.import_module() to import thread
   and threading modules
 * Add @unittest.skipUnless(threading, ...) to testcases using threads
diff --git a/Lib/test/test_poplib.py b/Lib/test/test_poplib.py
index f75f0b1..4375f71 100644
--- a/Lib/test/test_poplib.py
+++ b/Lib/test/test_poplib.py
@@ -4,7 +4,6 @@
 # a real test suite
 
 import poplib
-import threading
 import asyncore
 import asynchat
 import socket
@@ -15,6 +14,7 @@
 from unittest import TestCase
 from test import test_support
 from test.test_support import HOST
+threading = test_support.import_module('threading')
 
 
 # the dummy data returned by server when LIST and RETR commands are issued