fix tests in python 2

tensorpack/tfutils/unit_tests.py

 #-*- coding: utf-8 -*-
-#File:
 
+import six
 import unittest
 import tensorflow as tf
 
@@ -21,6 +21,7 @@ class ScopeUtilsTest(unittest.TestCase):
         with self.assertRaises(AssertionError):
             self._f()  # name conflict
 
+    @unittest.skipIf(six.PY2, "assertLogs not supported in Python 2")
     def test_under_name_scope_warning(self):
         x = tf.placeholder(tf.float32, [3])
         tf.nn.relu(x, name='s')