combine_maps: only use python2

python3 is not required on host machines.

Bug: 117260689
Test: m selinux_policy
Change-Id: Iea31eadcb3f09ffadca82dd1862cf5538259c710

tests/Android.bp

@@ -70,4 +70,5 @@ python_binary_host {
         "combine_maps.py",
         "mini_parser.py",
     ],
+    defaults: ["py2_only"],
 }