fix linting error

Author: charlesdong1991

pandas/tests/generic/test_frame.py

@@ -1,10 +1,10 @@
 # -*- coding: utf-8 -*-
 # pylint: disable-msg=E1101,W0612
 
-import os
 from copy import deepcopy
 from distutils.version import LooseVersion
 from operator import methodcaller
+import os
 
 import numpy as np
 import pytest