[py3] Refactored __unicode__ to __str__.

* Renamed the __unicode__ methods
* Applied the python_2_unicode_compatible decorator
* Removed the StrAndUnicode mix-in that is superseded by
  python_2_unicode_compatible
* Kept the __unicode__ methods in classes that specifically
  test it under Python 2

django/test/html.py

@@ -8,6 +8,7 @@ import re
 from django.utils.encoding import force_text
 from django.utils.html_parser import HTMLParser, HTMLParseError
 from django.utils import six
+from django.utils.encoding import python_2_unicode_compatible
 
 
 WHITESPACE = re.compile('\s+')
@@ -17,6 +18,7 @@ def normalize_whitespace(string):
     return WHITESPACE.sub(' ', string)
 
 
+@python_2_unicode_compatible
 class Element(object):
     def __init__(self, name, attributes):
         self.name = name
@@ -117,7 +119,7 @@ class Element(object):
     def __getitem__(self, key):
         return self.children[key]
 
-    def __unicode__(self):
+    def __str__(self):
         output = '<%s' % self.name
         for key, value in self.attributes:
             if value:
@@ -136,11 +138,12 @@ class Element(object):
         return six.text_type(self)
 
 
+@python_2_unicode_compatible
 class RootElement(Element):
     def __init__(self):
         super(RootElement, self).__init__(None, ())
 
-    def __unicode__(self):
+    def __str__(self):
         return ''.join(six.text_type(c) for c in self.children)