message: Update TF's six dependency.
diff: @@ -434,12 +434,12 @@ def tf_repositories(path_prefix = "", tf_repo_name = ""):
  tf_http_archive(
  name = "six_archive",
  build_file = clean_dep("//third_party:six.BUILD"),
  - sha256 = "d16a0141ec1a18405cd4ce8b4613101da75da0e9a7aec5bdd4fa804d0e0eba73",
  - strip_prefix = "six-1.12.0",
  + sha256 = "30639c035cdb23534cd4aa2dd52c3bf48f06e5f4a941509c8bafd8ce11080259",
  + strip_prefix = "six-1.15.0",
  system_build_file = clean_dep("//third_party/systemlibs:six.BUILD"),
  urls = [
  - "https://storage.googleapis.com/mirror.tensorflow.org/pypi.python.org/packages/source/s/six/six-1.12.0.tar.gz",
  - "https://pypi.python.org/packages/source/s/six/six-1.12.0.tar.gz",
  + "https://storage.googleapis.com/mirror.tensorflow.org/pypi.python.org/packages/source/s/six/six-1.15.0.tar.gz",
  + "https://pypi.python.org/packages/source/s/six/six-1.15.0.tar.gz",
  ],
  )
  