Fix CodeClimate duplication parser version

The plugin was already enabled by default, so this does not turn it on.
However, it also defaults to Python 2, which means it produces a lot of
syntax errors, and the run (silently) failed every time.

Author: jakelishman

.codeclimate.yml

@@ -38,3 +38,9 @@ plugins:
     enabled: true
   pep8:
     enabled: true
+  duplication:
+    enabled: true
+    config:
+      languages:
+        python:
+          python_version: 3