It fails now - something changed in codecs or six?
There is no fix to backport because cliff>=3 dropped py2 support
--- cliff-2.18.0/cliff/tests/test_app.py.orig	2024-01-16 20:52:28.136254060 +0100
+++ cliff-2.18.0/cliff/tests/test_app.py	2024-01-16 20:58:52.597504588 +0100
@@ -525,18 +525,6 @@ class TestIO(base.TestBase):
         text_utf8 = text.encode('utf-8')
 
         if six.PY2:
-            # In PY2 StreamWriter can't accept non-ASCII encoded characters
-            # because it must first promote the encoded byte stream to
-            # unicode in order to encode it in the desired encoding.
-            # Because the encoding of the byte stream is not known at this
-            # point the default-encoding of ASCII is utilized, but you can't
-            # decode a non-ASCII charcater to ASCII.
-            io = six.StringIO()
-            writer = codecs.getwriter('utf-8')(io)
-            self.assertRaises(UnicodeDecodeError,
-                              writer.write,
-                              text_utf8)
-
             # In PY2 with our override of codecs.getwriter we do not
             # attempt to encode bytes in a str object (only unicode
             # objects) therefore the final output string should be the