Remove six usage from freezerclient package

We don't support Python 2 anymore so we don't need this
compatibility library.

This patch remove six.moves.cStringIO by io.StringIO.

Change-Id: I31acfc0b661797fdefd23c28b06de6c21019de00

freezerclient/tests/unit/test_shell.py

@@ -17,7 +17,7 @@ import re
 import sys
 
 import fixtures
-import six
+import io
 import testtools
 from testtools import matchers
 
@@ -58,8 +58,8 @@ class ShellTest(testtools.TestCase):
         clean_env = {}
         _old_env, os.environ = os.environ, clean_env.copy()
         try:
-            sys.stdout = six.moves.cStringIO()
-            sys.stderr = six.moves.cStringIO()
+            sys.stdout = io.StringIO()
+            sys.stderr = io.StringIO()
             _shell = openstack_shell.FreezerShell()
             _shell.run(argstr.split())
         except SystemExit:

lower-constraints.txt

@@ -42,7 +42,6 @@ PyYAML==3.12
 reno==2.5.0
 requests==2.14.2
 setuptools==21.0.0
-six==1.10.0
 snowballstemmer==1.2.1
 Sphinx==1.6.5
 sphinxcontrib-websupport==1.0.1