Leverage dict comprehension in PEP-0274

PEP-0274 introduced dict comprehensions to replace dict constructor
with a sqeuence of key-pairs[1], these are benefits:
  First, it makes the code look neater.
  Second, it gains a micro-optimization.

We dropped python 2.6 support, we can leverage this now.
Note: This commit doesn't handle dict constructor with kwargs.

[1]http://legacy.python.org/dev/peps/pep-0274/

Change-Id: Ifca656d516809a6a52cf15bac3f6361e8d83aa58

oslo_versionedobjects/exception.py

@@ -55,7 +55,7 @@ class ConvertedException(webob.exc.WSGIHTTPException):
 
 def _cleanse_dict(original):
     """Strip all admin_password, new_pass, rescue_pass keys from a dict."""
-    return dict([(k, v) for k, v in original.items() if "_pass" not in k])
+    return {k: v for k, v in original.items() if "_pass" not in k}
 
 
 def wrap_exception(notifier=None, get_notifier=None):

oslo_versionedobjects/fields.py

@@ -525,16 +525,15 @@ class DictProxyField(object):
             return self
         if getattr(obj, self._fld_name) is None:
             return
-        return dict([(self._key_type(k), v)
-                     for k, v in six.iteritems(getattr(obj, self._fld_name))])
+        return {self._key_type(k): v
+                for k, v in six.iteritems(getattr(obj, self._fld_name))}
 
     def __set__(self, obj, val):
         if val is None:
             setattr(obj, self._fld_name, val)
         else:
             setattr(obj, self._fld_name,
-                    dict([(six.text_type(k), v)
-                          for k, v in six.iteritems(val)]))
+                    {six.text_type(k): v for k, v in six.iteritems(val)})
 
 
 class Set(CompoundFieldType):