Try to import MutableMapping from collections.abc (#403)

Note that collections.abc has been added in Python 3.3.

Fixes #402

Author: 5j9

html5lib/_trie/_base.py

@@ -1,6 +1,9 @@
 from __future__ import absolute_import, division, unicode_literals
 
-from collections import Mapping
+try:
+    from collections.abc import Mapping
+except ImportError:  # Python 2.7
+    from collections import Mapping
 
 
 class Trie(Mapping):

html5lib/treebuilders/dom.py

@@ -1,7 +1,10 @@
 from __future__ import absolute_import, division, unicode_literals
 
 
-from collections import MutableMapping
+try:
+    from collections.abc import MutableMapping
+except ImportError:  # Python 2.7
+    from collections import MutableMapping
 from xml.dom import minidom, Node
 import weakref