Fixed #18013 -- Use the new 'as' syntax for exceptions.

Thanks Clueless for the initial patch.
Note that unittest has been purposely left out (external package only used by Python 2.6).

django/utils/encoding.py

@@ -88,7 +88,7 @@ def force_unicode(s, encoding='utf-8', strings_only=False, errors='strict'):
             # errors), so that if s is a SafeString, it ends up being a
             # SafeUnicode at the end.
             s = s.decode(encoding, errors)
-    except UnicodeDecodeError, e:
+    except UnicodeDecodeError as e:
         if not isinstance(s, Exception):
             raise DjangoUnicodeDecodeError(s, *e.args)
         else: