chg: Force pickle to use highest protocol

Python2.7 uses protocol 1 by default, which is not compatible with
class defining __slots__. On the other hand, all the Python versions
support versions >= 2.

lark/lark.py

@@ -336,7 +336,8 @@ class Lark(Serialize):
         Useful for caching and multiprocessing.
         """
         data, m = self.memo_serialize([TerminalDef, Rule])
-        pickle.dump({'data': data, 'memo': m}, f)
+        pickle.dump({'data': data, 'memo': m}, f,
+                    protocol=pickle.HIGHEST_PROTOCOL)
 
     @classmethod
     def load(cls, f):

tests/test_cache.py

@@ -93,6 +93,3 @@ class TestCache(TestCase):
 
 if __name__ == '__main__':
     main()
-
-
-

tests/test_trees.py

@@ -20,7 +20,7 @@ class TestTrees(TestCase):
 
     def test_pickle(self):
         s = copy.deepcopy(self.tree1)
-        data = pickle.dumps(s)
+        data = pickle.dumps(s, protocol=pickle.HIGHEST_PROTOCOL)
         assert pickle.loads(data) == s
 
     def test_repr_runnable(self):