compare-results.py: use python3, python is no longer available in our build images

Change-Id: I268980c8a2b0a4e826c0c488ce6315462c01cabb
diff --git a/compare-results.py b/compare-results.py
index d1adb20..556a063 100755
--- a/compare-results.py
+++ b/compare-results.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright 2018 sysmocom - s.f.m.c. GmbH
 #
 # Licensed under the Apache License, Version 2.0 (the "License");