Fixes doctest for python3

Dictionary hash are dynamic in Python3 so the __repr__ output will be different all the time. Doctest should check for value equality instead. 

Fixes issue on https://nltk.ci.cloudbees.com/job/nltk/TOXENV=py34-jenkins,jdk=jdk8latestOnlineInstall/lastCompletedBuild/testReport/nltk.tokenize.mwe/MWETokenizer/add_mwe/

nltk/tokenize/mwe.py

@@ -70,8 +70,9 @@ def add_mwe(self, mwe):
         >>> tokenizer.add_mwe(('a', 'b'))
         >>> tokenizer.add_mwe(('a', 'b', 'c'))
         >>> tokenizer.add_mwe(('a', 'x'))
-        >>> tokenizer._mwes.as_dict()
-        {'a': {'x': {True: None}, 'b': {True: None, 'c': {True: None}}}}
+        >>> expected = {'a': {'x': {True: None}, 'b': {True: None, 'c': {True: None}}}}
+        >>> tokenizer._mwes.as_dict() == expected
+        True
 
         """
         self._mwes.insert(mwe)