Switch genrules from tools to exec_tools for python binaries.

This is required to use py3 targets as part of a genrule; cf:
bazelbuild/bazel#6443

PiperOrigin-RevId: 319104056

tensorflow_estimator/python/estimator/api/api_gen.bzl

@@ -120,6 +120,6 @@ def gen_api_init_files(
             " --output_package=" + output_package + " $(OUTS)"
         ),
         srcs = srcs,
-        tools = [":" + api_gen_binary_target],
+        exec_tools = [":" + api_gen_binary_target],
         visibility = ["//visibility:public"],
     )