Fix941 Triangle: add topics (#944)

* issue fixed

* minor bug fixed

* #941 fixed

* #941 fixed

* triangle: topics added

config.json

       "core": true,
       "difficulty": 4,
       "slug": "triangle",
-      "topics": null,
+      "topics": [
+        "booleans",
+        "classes",
+        "exception_handling",
+        "mathematics"
+      ],
       "unlocked_by": null,
       "uuid": "ec268d8e-997b-4553-8c67-8bdfa1ecb888"
     },