Followup for grouping imports

This patch closes the comment [1], it groups the imports in
test_rbac_rule_validation.py

[1]https://review.opendev.org/#/c/720971/1/patrole_tempest_plugin/tests/unit/test_rbac_rule_validation.py@22

Change-Id: I916796933dc4cee84229850ad87ea6190254d548
diff --git a/patrole_tempest_plugin/tests/unit/test_rbac_rule_validation.py b/patrole_tempest_plugin/tests/unit/test_rbac_rule_validation.py
index 2c7307e..b2edfb4 100644
--- a/patrole_tempest_plugin/tests/unit/test_rbac_rule_validation.py
+++ b/patrole_tempest_plugin/tests/unit/test_rbac_rule_validation.py
@@ -15,10 +15,9 @@
 from __future__ import absolute_import
 from unittest import mock
 
+import fixtures
 import functools
 from oslo_config import cfg
-
-import fixtures
 from tempest.lib import exceptions
 
 from patrole_tempest_plugin import rbac_exceptions