import "collections.abc" explicitly

Collections Abstract Base Classes have been moved to the
collections.abc module, which was part of the collections
module before[1] [2]. Usage from the collections module
directly is deprecated and will be removed from 3.10.
This commit updates to import abc module explicitly
wherever required as the same will no longer be visible/directly
used from the collections module.

[1] https://docs.python.org/3/library/collections.abc.html
[2] http://paste.openstack.org/show/802512/

Change-Id: I173f4d647474c21c5d062280bd5703edffb4978e

horizon/base.py

@@ -21,6 +21,7 @@ the classes contained therein.
 """
 
 import collections
+import collections.abc
 import copy
 from importlib import import_module
 import inspect

horizon/tables/base.py

@@ -13,6 +13,7 @@
 #    under the License.
 
 import collections
+import collections.abc
 import copy
 import inspect
 import json

horizon/test/helpers.py

@@ -16,7 +16,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import collections
+import collections.abc
 import copy
 import logging
 import os

openstack_dashboard/test/integration_tests/decorators.py

@@ -10,7 +10,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import collections
+import collections.abc
 import functools
 import inspect