fix delete last label disassociate

* Inherit from mixin first in order to overwrite next inherited class
method, unattach.
* Also, fix up test and add test to ensure correct inheritance

awx/main/tests/unit/api/test_views.py

@@ -9,6 +9,7 @@ from django.contrib.contenttypes.models import ContentType
 from awx.api.views import (
     ApiV1RootView,
     TeamRolesList,
+    JobTemplateLabelList,
 )
 
 from awx.main.models import (
@@ -62,6 +63,15 @@ class TestApiV1RootView:
         for endpoint in endpoints:
             assert endpoint in data_arg
 
+class TestJobTemplateLabelList:
+    def test_inherited_mixin_unattach(self):
+        with mock.patch('awx.api.generics.DeleteLastUnattachLabelMixin.unattach') as mixin_unattach:
+            view = JobTemplateLabelList()
+            mock_request = mock.MagicMock()
+            
+            super(JobTemplateLabelList, view).unattach(mock_request, None, None)
+            assert mixin_unattach.called_with(mock_request, None, None)
+
 @pytest.mark.parametrize("url", ["/team/1/roles", "/role/1/teams"])
 def test_team_roles_list_post_org_roles(url):
     with mock.patch('awx.api.views.Role.objects.get') as role_get, \
@@ -84,3 +94,4 @@ def test_team_roles_list_post_org_roles(url):
 
         assert response.status_code == 400
         assert 'cannot assign' in response.content
+