[py3] Added fixer for python_2_unicode_compatible.

This doesn't deal with classes that define both __unicode__ and
__str__; the definition of __str__ should be removed first. It
doesn't guarantee that __str__ will return a str (rather than bytes)
under Python 3 either.

django/bin/django-2to3.py

@@ -0,0 +1,10 @@
+#!/usr/bin/env python
+
+# This works exactly like 2to3, except that it uses Django's fixers rather
+# than 2to3's built-in fixers.
+
+import sys
+from lib2to3.main import main
+
+sys.exit(main("django.utils.2to3_fixes"))
+

django/utils/2to3_fixes/fix_unicode.py

@@ -0,0 +1,36 @@
+"""Fixer for __unicode__ methods.
+
+Uses the django.utils.encoding.python_2_unicode_compatible decorator.
+"""
+
+from __future__ import unicode_literals
+
+from lib2to3 import fixer_base
+from lib2to3.fixer_util import find_indentation, Name, syms, touch_import
+from lib2to3.pgen2 import token
+from lib2to3.pytree import Leaf, Node
+
+
+class FixUnicode(fixer_base.BaseFix):
+
+    BM_compatible = True
+    PATTERN = """
+    classdef< 'class' any+ ':'
+              suite< any*
+                     funcdef< 'def' unifunc='__unicode__'
+                              parameters< '(' NAME ')' > any+ >
+                     any* > >
+    """
+
+    def transform(self, node, results):
+        unifunc = results["unifunc"]
+        strfunc = Name("__str__", prefix=unifunc.prefix)
+        unifunc.replace(strfunc)
+
+        klass = node.clone()
+        klass.prefix = '\n' + find_indentation(node)
+        decorator = Node(syms.decorator, [Leaf(token.AT, "@"), Name('python_2_unicode_compatible')])
+        decorated = Node(syms.decorated, [decorator, klass], prefix=node.prefix)
+        node.replace(decorated)
+
+        touch_import('django.utils.encoding', 'python_2_unicode_compatible', decorated)