message: Update protobuf for Python 3.7 compat
  Regards tensorflow/tensorflow#20517
diff: @@ -345,11 +345,11 @@ def tf_workspace(path_prefix = "", tf_repo_name = ""):
  )
  PROTOBUF_URLS = [
  - "https://mirror.bazel.build/github.com/google/protobuf/archive/v3.6.0.tar.gz",
  - "https://github.com/google/protobuf/archive/v3.6.0.tar.gz",
  + "https://mirror.bazel.build/github.com/google/protobuf/archive/v3.6.1.tar.gz",
  + "https://github.com/google/protobuf/archive/v3.6.1.tar.gz",
  ]
  - PROTOBUF_SHA256 = "50a5753995b3142627ac55cfd496cebc418a2e575ca0236e29033c67bd5665f4"
  - PROTOBUF_STRIP_PREFIX = "protobuf-3.6.0"
  + PROTOBUF_SHA256 = "3d4e589d81b2006ca603c1ab712c9715a76227293032d05b26fca603f90b3f5b"
  + PROTOBUF_STRIP_PREFIX = "protobuf-3.6.1"
  tf_http_archive(
  name = "protobuf_archive",
  