Merge for issue #15744: add tests for the writelines() method of file objects.

diff --cc Lib/test/test_fileio.py
Simple merge

diff --cc Lib/test/test_io.py
index d5eec7c2215f7b835628643bb4de9e0f7d7715de,56ac2c8a792b18d17602040ed105e23bb596a43a..555ad7444709acb450da31763ef9cf17c613d636
--- 1/Lib/test/test_io.py
--- 2/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@@ -19,21 -19,20 +19,21 @@@
  # test both implementations. This file has lots of examples.
  ################################################################################
  
 +import abc
 +import array
 +import errno
 +import locale
  import os
 +import pickle
 +import random
 +import signal
  import sys
  import time
 -import array
 -import random
  import unittest
 -import weakref
 -import abc
 -import signal
 -import errno
  import warnings
 -import pickle
 -from itertools import cycle, count
 +import weakref
- from collections import deque
+ from collections import deque, UserList
 +from itertools import cycle, count
  from test import support
  
  import codecs