Use yaml.safe_load()

Pyyaml>=6.0 requires to pass the Loader arg to yaml.load(), switching to
yaml.safe_load() recovers the old and expected behavior.

yaml/pyyaml#561

Closes-Bug: #1951650

zaza/openstack/charm_tests/neutron/tests.py

@@ -285,7 +285,7 @@ def _assert_result_match(self, action_result, resource_list,
 
         # extract data from juju action
         action_data = action_result.data.get('results', {}).get(resource_name)
-        resources_from_action = yaml.load(action_data)
+        resources_from_action = yaml.safe_load(action_data)
 
         # pull resource IDs from expected resource list and juju action data
         expected_resource_ids = {resource['id'] for resource in resource_list}