Remove usage of six

Remove six-library, replace the following items with Python 3 style
code:
- six.moves
- six.iteritems

Remove six-library in requirement.txt

Change-Id: Ic45dff7406937c9ee6d7bd2286107bcd33e5a16d
diff --git a/tools/check_logs.py b/tools/check_logs.py
index cc74b17..8ab3af2 100755
--- a/tools/check_logs.py
+++ b/tools/check_logs.py
@@ -23,7 +23,6 @@
 import sys
 import urllib.request as urlreq
 
-import six
 import yaml
 
 # DEVSTACK_GATE_GRENADE is either unset if grenade is not running
@@ -137,7 +136,7 @@
     with open(ALLOW_LIST_FILE) as stream:
         loaded = yaml.safe_load(stream)
         if loaded:
-            for (name, l) in six.iteritems(loaded):
+            for (name, l) in loaded.values():
                 for w in l:
                     assert 'module' in w, 'no module in %s' % name
                     assert 'message' in w, 'no message in %s' % name