Fixed #21198 -- Prevented invalid use of @python_2_unicode_compatible.

Thanks jpic for the report and chmodas for working on a patch.

Reverts 2ea80b94. Refs #19362.

django/utils/encoding.py

@@ -28,6 +28,10 @@ def python_2_unicode_compatible(klass):
     returning text and apply this decorator to the class.
     """
     if six.PY2:
+        if '__str__' not in klass.__dict__:
+            raise ValueError("@python_2_unicode_compatible cannot be applied "
+                             "to %s because it doesn't define __str__()." %
+                             klass.__name__)
         klass.__unicode__ = klass.__str__
         klass.__str__ = lambda self: self.__unicode__().encode('utf-8')
     return klass