Change demo file compatible to python2 (#40)

* Change demo file to python2 compatible

* Add object to classes for compatibility with python2

scripts/algorithms/common/buffer/replay_buffer.py

@@ -11,7 +11,7 @@ from algorithms.common.helper_functions import get_n_step_info
 device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")
 
 
-class ReplayBuffer:
+class ReplayBuffer(object):
     """Fixed-size buffer to store experience tuples.
 
     Taken from Udacity deep-reinforcement-learning github repository:
@@ -79,7 +79,7 @@ class ReplayBuffer:
         return len(self.buffer)
 
 
-class NStepTransitionBuffer:
+class NStepTransitionBuffer(object):
     """Fixed-size buffer to store experience tuples.
 
     Attributes:

scripts/algorithms/common/buffer/segment_tree.py

@@ -4,7 +4,7 @@
 import operator
 
 
-class SegmentTree:
+class SegmentTree(object):
     """ Create SegmentTree.
 
     Taken from OpenAI baselines github repository: