make FileIO.mode always include 'b'

#4386 Reviewed by Amaury
diff --git a/Lib/test/test_socket.py b/Lib/test/test_socket.py
index b2323fe..51f41a9 100644
--- a/Lib/test/test_socket.py
+++ b/Lib/test/test_socket.py
@@ -849,11 +849,11 @@
         self.assert_(not self.cli_file.closed)
 
     def testAttributes(self):
-        self.assertEqual(self.serv_file.mode, 'r')
+        self.assertEqual(self.serv_file.mode, 'rb')
         self.assertEqual(self.serv_file.name, self.cli_conn.fileno())
 
     def _testAttributes(self):
-        self.assertEqual(self.cli_file.mode, 'w')
+        self.assertEqual(self.cli_file.mode, 'wb')
         self.assertEqual(self.cli_file.name, self.serv_conn.fileno())
 
 class UnbufferedFileObjectClassTestCase(FileObjectClassTestCase):