Remove Python 2-specific imports

Change-Id: I64810898cd9126cf619df0b8f60e6fa01958943e
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

nova/scheduler/host_manager.py

@@ -20,11 +20,6 @@ Manage hosts in the current zone.
 import collections
 import functools
 import time
-try:
-    from collections import UserDict as IterableUserDict   # Python 3
-except ImportError:
-    from UserDict import IterableUserDict                  # Python 2
-
 
 import iso8601
 from oslo_log import log as logging
@@ -47,7 +42,7 @@ LOG = logging.getLogger(__name__)
 HOST_INSTANCE_SEMAPHORE = "host_instance"
 
 
-class ReadOnlyDict(IterableUserDict):
+class ReadOnlyDict(collections.UserDict):
     """A read-only dict."""
 
     def __init__(self, source=None):

nova/tests/unit/test_metadata.py

@@ -20,13 +20,9 @@ import copy
 import hashlib
 import hmac
 import os
+import pickle
 import re
 
-try:  # python 2
-    import pickle
-except ImportError:  # python 3
-    import cPickle as pickle
-
 from keystoneauth1 import exceptions as ks_exceptions
 from keystoneauth1 import session
 import mock