> Use str instead.

Sure. I understand the advantages of the new approach to strings. Which, by the way, weren't available when this project began. I don't disagree with anything you say in the context of writing new code. I was, however, surprised and dismayed to learn of the cavalier approach the upgrade tool has taken to silently breaking existing code which it is claiming to "fix."

Here's my favorite so far.

--- cdr.py      (original)
+++ cdr.py      (refactored)
@@ -36,15 +36,15 @@
 # ======================================================================
 from six import itervalues
 try:
-    basestring
+    str
     is_python3 = False
     base64encode = base64.encodestring
     base64decode = base64.decodestring
 except:
     base64encode = base64.encodebytes
     base64decode = base64.decodebytes
-    basestring = (str, bytes)
-    unicode = str
+    str = (str, bytes)
+    str = str
     is_python3 = True

We wrote this following the example of comparable techniques in http://python-future.org/compatible_idioms.html and similar guides to an upgrade path. Seems we're being punished for taking the trouble to make our code work with Python 2 and 3 during the transition period. :-(

It's hard to see how this conversion resulted in something better than what we already had.