convert to python3

Index: make_palettes.py
--- make_palettes.py.orig
+++ make_palettes.py
@@ -11,7 +11,7 @@ palettes = set()
 out = open("palettes.cpp", "w")
 
 def die(msg):
-    print "Failed: %s" % msg
+    print("Failed: %s" % msg)
     sys.exit(1)
 
 # what variable name should we use for the given base filename