[py3] Ported django.utils.encoding.

* Renamed smart_unicode to smart_text (but kept the old name under
  Python 2 for backwards compatibility).
* Renamed smart_str to smart_bytes.
* Re-introduced smart_str as an alias for smart_text under Python 3
  and smart_bytes under Python 2 (which is backwards compatible).
  Thus smart_str always returns a str objects.
* Used the new smart_str in a few places where both Python 2 and 3
  want a str.

django/test/html.py

@@ -5,7 +5,7 @@ Comparing two html documents.
 from __future__ import unicode_literals
 
 import re
-from django.utils.encoding import force_unicode
+from django.utils.encoding import force_text
 from django.utils.html_parser import HTMLParser, HTMLParseError
 from django.utils import six
 
@@ -25,7 +25,7 @@ class Element(object):
 
     def append(self, element):
         if isinstance(element, six.string_types):
-            element = force_unicode(element)
+            element = force_text(element)
             element = normalize_whitespace(element)
             if self.children:
                 if isinstance(self.children[-1], six.string_types):