Add third_party/rules_python to WORKSPACE as @rules_python

These will be used by third_party/protobuf in a follow-up.

Also update a test that uses protobuf and that will break when protobuf is
updated (since the test can't be changed in the same PR as third_party/).

Break-out of #9019. Work toward #9006.

RELNOTES: None

WORKSPACE

@@ -79,6 +79,16 @@ new_local_repository(
     path = "./third_party/protobuf/3.6.1/",
 )
 
+# This is a mock version of bazelbuild/rules_python that contains only
+# @rules_python//python:defs.bzl. It is used by protobuf.
+# TODO(#9029): We could potentially replace this with the real @rules_python.
+new_local_repository(
+    name = "rules_python",
+    path = "./third_party/rules_python",
+    build_file = "//third_party/rules_python:BUILD",
+    workspace_file = "//third_party/rules_python:rules_python.WORKSPACE",
+)
+
 local_repository(
     name = "googleapis",
     path = "./third_party/googleapis/",

src/test/shell/integration/modify_execution_info_test.sh

@@ -158,6 +158,16 @@ new_local_repository(
     path = "$(dirname $(rlocation io_bazel/third_party/protobuf/3.6.1/BUILD))",
     build_file = "$(rlocation io_bazel/third_party/protobuf/3.6.1/BUILD)",
 )
+
+# TODO(#9029): May require some adjustment if/when we depend on the real
+# @rules_python in the real source tree, since this third_party/ package won't
+# be available.
+new_local_repository(
+    name = "rules_python",
+    path = "$(dirname $(rlocation io_bazel/third_party/rules_python/rules_python.WORKSPACE))",
+    build_file = "$(rlocation io_bazel/third_party/rules_python/BUILD)",
+    workspace_file = "$(rlocation io_bazel/third_party/rules_python/rules_python.WORKSPACE)",
+)
 EOF
   fi
   local pkg="${FUNCNAME[0]}"