Replace six.iteritems() with .items()

1.As mentioned in [1], we should avoid usingg
six.iteritems to achieve iterators. We can
use dict.items instead, as it will return
iterators in PY3 as well. And dict.items/keys
will more readable. 2.In py2, the performance
about list should be negligible, see the link [2].
[1] https://wiki.openstack.org/wiki/Python3
[2] http://lists.openstack.org/pipermail/openstack-dev/2015-June/066391.html

Change-Id: I71c13040318eca6e5ed993e8aa03f8003986a71c

glanceclient/common/http.py

@@ -53,7 +53,7 @@ def encode_headers(headers):
               names and values
     """
     return dict((encodeutils.safe_encode(h), encodeutils.safe_encode(v))
-                for h, v in six.iteritems(headers) if v is not None)
+                for h, v in headers.items() if v is not None)
 
 
 class _BaseHTTPClient(object):
@@ -181,7 +181,7 @@ class HTTPClient(_BaseHTTPClient):
         headers = copy.deepcopy(headers)
         headers.update(self.session.headers)
 
-        for (key, value) in six.iteritems(headers):
+        for (key, value) in headers.items():
             header = '-H \'%s: %s\'' % utils.safe_header(key, value)
             curl.append(header)
 
@@ -227,7 +227,7 @@ class HTTPClient(_BaseHTTPClient):
         headers = copy.deepcopy(kwargs.pop('headers', {}))
 
         if self.identity_headers:
-            for k, v in six.iteritems(self.identity_headers):
+            for k, v in self.identity_headers.items():
                 headers.setdefault(k, v)
 
         data = self._set_common_request_kwargs(headers, kwargs)