Fix DATrie support under Python 2.

This is a simple issue of using `str` to refer to what should be
`six.text_type`.

Author: gsnedders

html5lib/trie/datrie.py

@@ -3,14 +3,15 @@
 from itertools import chain
 
 from datrie import Trie as DATrie
+from six import text_type
 
 from ._base import Trie as ABCTrie
 
 class Trie(ABCTrie):
     def __init__(self, data):
         chars = set()
         for key in data.keys():
-            if not isinstance(key, str):
+            if not isinstance(key, text_type):
                 raise TypeError("All keys must be strings")
             for char in key:
                 chars.add(char)