os.sendfile() test: fix "AttributeError: 'NoneType' object has no attribute…

os.sendfile() test: fix "AttributeError: 'NoneType' object has no attribute 'Thread'" when running tests with --without-threads option.
reference: http://bugs.python.org/issue10882#msg136257

Lib/test/test_os.py

@@ -1309,7 +1309,8 @@ class ProgramPriorityTests(unittest.TestCase):
                     raise
 
 
-class SendfileTestServer(asyncore.dispatcher, threading.Thread):
+if threading is not None:
+    class SendfileTestServer(asyncore.dispatcher, threading.Thread):
 
         class Handler(asynchat.async_chat):