Fix failing test in Python 3

IPython/utils/tests/test_tempdir.py

@@ -1,20 +1,20 b''
 #-----------------------------------------------------------------------------
 #  Copyright (C) 2012-  The IPython Development Team
 #
 #  Distributed under the terms of the BSD License.  The full license is in
 #  the file COPYING, distributed as part of this software.
 #-----------------------------------------------------------------------------
 
 import os
 
 from IPython.utils.tempdir import NamedFileInTemporaryDirectory
 
 
 def test_named_file_in_temporary_directory():
     with NamedFileInTemporaryDirectory('filename') as file:
         name = file.name
         assert not file.closed
         assert os.path.exists(name)
-        file.write('test')
+        file.write(b'test')
     assert file.closed
     assert not os.path.exists(name)