gh-125196: Use PyUnicodeWriter for repr(structseq) (#125219)

Replace the private _PyUnicodeWriter with the public PyUnicodeWriter.

* Avoid temporary PyUnicode_DecodeUTF8(): call
  PyUnicodeWriter_WriteUTF8() instead.
* Avoid temporary PyObject_Repr(): call PyUnicodeWriter_WriteRepr()
  instead.