tp: switch merged metrics script to python3

Change-Id: I2765179dc7851b6a5c59c4a166c1e06be8beda32
diff --git a/BUILD.extras b/BUILD.extras
index 6365f5c..6a0c30e 100644
--- a/BUILD.extras
+++ b/BUILD.extras
@@ -19,7 +19,7 @@
         "tools/gen_merged_sql_metrics.py",
     ],
     main = "tools/gen_merged_sql_metrics.py",
-    python_version = "PY2",
+    python_version = "PY3",
 )
 
 perfetto_java_proto_library(