message: Prefer exec_tool for Python tools to ensure PY3 compat
diff: @@ -72,7 +72,7 @@ def generate_flex_kernel_header(
  name = "%s_kernel_registration" % name,
  srcs = [list_ops_output],
  outs = [header],
  - tools = [tool],
  + exec_tools = [tool],
  message = "Processing %s..." % list_ops_output,
  cmd = ("$(location " + tool + ")" +
  " --default_ops=\"\"" +
  