Tell LGTM to use Python 3 explicitly.

I'm not sure if this affects the Python analysis (it might as there are
some strange 'issues'), but the C/C++ analysis is definitely broken due
to using Python 2 to build.

Author: QuLogic

.lgtm.yml

@@ -1,3 +1,7 @@
+extraction:
+  python:
+    python_setup:
+      version: 3
 path_classifiers:
     examples:
         # Exclude example files from analysis by tagging them.