Remove unused six and missing six import

python/paddle/dataset/mnist.py

@@ -24,7 +24,6 @@ import paddle.dataset.common
 import subprocess
 import numpy
 import platform
-import six
 import tempfile
 from six.moves import range
 __all__ = ['train', 'test', 'convert']

python/paddle/fluid/tests/unittests/dist_se_resnext.py

@@ -16,7 +16,6 @@ from __future__ import print_function
 
 import numpy as np
 import argparse
-import six
 import time
 import math
 

python/paddle/fluid/transpiler/distribute_transpiler.py

@@ -34,6 +34,7 @@ import math
 import random
 import numpy as np
 import collections
+import six
 
 from .ps_dispatcher import RoundRobin, HashName, PSDispatcher
 from .. import core, framework