Merge pull request #886 from puripant/patch-1

Add topics to sieve

config.json

       "core": false,
       "difficulty": 4,
       "slug": "sieve",
-      "topics": null,
+      "topics": [
+        "loops",
+        "lists",
+        "integers",
+        "algorithms",
+        "mathematics"
+      ],
       "unlocked_by": "difference-of-squares",
       "uuid": "6791d01f-bae4-4b63-ae76-86529ac49e36"
     },