Add topic to pythagorean-triplet (#969)

* Add topic to pythagorean-triplet

* cr feedback

* Update config.json

* Update config.json

Attempt to fix space issue.

config.json

       "core": false,
       "difficulty": 9,
       "slug": "pythagorean-triplet",
-      "topics": null,
+      "topics": [
+       "integer",
+	"lists",
+	"logic",
+	"mathematics"
+      ],
       "unlocked_by": "triangle",
       "uuid": "88505f95-89e5-4a08-8ed2-208eb818cdf1"
     },