Drop remaining usage of six

It's not listed in requirements. Because python 2 reached its EOL some
time ago, python 2 compatibility may no longer be needed.

Change-Id: I04ff0b185d8c934867cf0012310289bb18af2eb8

unit_tests/test_cinder_hooks.py

@@ -12,11 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from importlib import reload
 import os
 import json
 
-from six.moves import reload_module
-
 from unittest.mock import (
     patch,
     call
@@ -34,7 +33,7 @@ with patch('charmhelpers.contrib.hardening.harden.harden') as mock_dec:
         with patch('cinder_utils.restart_map') as restart_map:
             restart_map.return_value = RESTART_MAP
             import cinder_hooks as hooks
-            reload_module(hooks)
+            reload(hooks)
 
 hooks.hooks._config_save = False
 import cinder_utils as utils