Make test_cache python 3 compatible

python/test/dune/perftool/generation/test_cache.py

@@ -11,7 +11,7 @@ from dune.perftool.generation.cache import(delete_cache_items,
 def print_cache():
     """Print all cache items"""
     for i in retrieve_cache_items("True"):
-        print i
+        print(i)
 
 
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