[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/utils/_os.py

@@ -1,7 +1,7 @@
 import os
 import stat
 from os.path import join, normcase, normpath, abspath, isabs, sep
-from django.utils.encoding import force_unicode
+from django.utils.encoding import force_text
 
 try:
     WindowsError = WindowsError
@@ -37,8 +37,8 @@ def safe_join(base, *paths):
     The final path must be located inside of the base path component (otherwise
     a ValueError is raised).
     """
-    base = force_unicode(base)
-    paths = [force_unicode(p) for p in paths]
+    base = force_text(base)
+    paths = [force_text(p) for p in paths]
     final_path = abspathu(join(base, *paths))
     base_path = abspathu(base)
     base_path_len = len(base_path)