fix: Fix support for many-to-many rels (#4)

Author: pikhovkin

schema_viewer/tests/test_schema.py

@@ -9,7 +9,10 @@ def test_schema(self):
             'auth_permission',
             'django_admin_log',
             'auth_group',
+            'auth_group_permissions',
             'auth_user',
+            'auth_user_user_permissions',
+            'auth_user_groups',
             'django_content_type',
             'django_session',
         }

schema_viewer/tests/test_view.py

@@ -13,7 +13,10 @@ def test_schema(self):
             'auth_permission',
             'django_admin_log',
             'auth_group',
+            'auth_group_permissions',
             'auth_user',
+            'auth_user_user_permissions',
+            'auth_user_groups',
             'django_content_type',
             'django_session',
         }