Update from https://crrev.com/308331

mojo/python/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//third_party/cython/rules.gni")
 
 # GYP version: mojo/mojo.gyp:mojo_python
 group("python") {
   deps = [
     ":embedder",
     ":validation_util",
     "//mojo/public/python",
   ]
 }
 
 # GYP version: mojo/mojo.gyp:mojo_python_embedder
 python_binary_module("embedder") {
   python_base_module = "mojo"
-  sources = [
+  cython_sources = [
     "system/mojo/embedder.pyx",
   ]
   deps = [
     "//mojo/edk/system",
   ]
   datadeps = [
     "//mojo/public/python:system",
   ]
 }
 
 copy("tests_module") {
   sources = [
     "system/mojo/tests/__init__.py",
   ]
   outputs = [
     "$root_out_dir/python/mojo/tests/{{source_file_part}}",
   ]
 }
 
 python_binary_module("validation_util") {
   python_base_module = "mojo/tests"
-  sources = [
+  cython_sources = [
     "system/mojo/tests/validation_util.pyx",
   ]
   deps = [
     "//mojo/public/cpp/bindings/tests:mojo_public_bindings_test_utils",
   ]
   datadeps = [
     ":tests_module",
   ]
 }