Enable python3 on some LUCI builds.

Test migration from python2 to python3.

Change-Id: Ic494a81328951650d71303451fc12a34392901a7
diff --git a/tools/luci/config/main.star b/tools/luci/config/main.star
index 714d652..ec1c88e 100755
--- a/tools/luci/config/main.star
+++ b/tools/luci/config/main.star
@@ -22,6 +22,10 @@
 
 lucicfg.check_version("1.30.9", "Please update depot_tools")
 
+luci.builder.defaults.experiments.set({
+    "luci.recipes.use_python3": 10,
+})
+
 # Use LUCI Scheduler BBv2 names and add Scheduler realms configs.
 lucicfg.enable_experiment("crbug.com/1182002")