message: Updated python/ to use tstring.
  This is a part of a larger migration effort for tensorflow::tstring.
  See:
diff: @@ -80,15 +80,15 @@ PyObject* ZeroDimArrayToScalar(PyObject* obj) {
  // Converts Python object `c` that should hold a Python string into a
  // C++ string in *out. Returns nullptr on success, or a message on error.
  // Defined below, but forward declared here for use in PyRepr.
  -const char* ConvertOneString(PyObject* v, string* out);
  +const char* ConvertOneString(PyObject* v, tstring* out);
  -string PyRepr(PyObject* obj) {
  +tstring PyRepr(PyObject* obj) {
  if (obj == nullptr) {
  return "<null>";
  }
  Safe_PyObjectPtr repr_obj = make_safe(PyObject_Repr(obj));
  if (repr_obj) {
  - string repr_str;
  + tstring repr_str;
  if (ConvertOneString(repr_obj.get(), &repr_str) == nullptr) {
  return repr_str;
  }
  @@ -446,7 +446,7 @@ DEFINE_HELPER(ConvertNumpyHalf, Eigen::half, DT_HALF, ConvertOneNumpyHalf);
  // String support
  -const char* ConvertOneString(PyObject* v, string* out) {
  +const char* ConvertOneString(PyObject* v, tstring* out) {
  if (PyBytes_Check(v)) {
  out->assign(PyBytes_AS_STRING(v), PyBytes_GET_SIZE(v));
  return nullptr;
  @@ -469,7 +469,7 @@ const char* ConvertOneString(PyObject* v, string* out) {
  return ErrorMixedTypes;
  }
  -DEFINE_HELPER(ConvertString, string, DT_STRING, ConvertOneString);
  +DEFINE_HELPER(ConvertString, tstring, DT_STRING, ConvertOneString);
  // Complex support
  