Iterate over copy of sys.modules keys in Python2/3

Iterate over a copy of sys.modules keys in both Python 2.x and
Python 3.x.  In Python 3.x, keys() is not a copy, and therefore
items can't be popped from it while iterating.

Change-Id: I98c3d7695bbfe3a6a4f23990af45a07dc147f22f
Closes-Bug: #1463503

keystoneclient/tests/unit/utils.py

@@ -179,7 +179,7 @@ class DisableModuleFixture(fixtures.Fixture):
 
     def clear_module(self):
         cleared_modules = {}
-        for fullname in sys.modules.keys():
+        for fullname in list(sys.modules):
             if (fullname == self.module or
                     fullname.startswith(self.module + '.')):
                 cleared_modules[fullname] = sys.modules.pop(fullname)