gh-127787: refactor helpers for PyUnicodeErrorObject internal interface

Include/cpython/pyerrors.h

@@ -94,6 +94,12 @@ PyAPI_FUNC(void) _PyErr_ChainExceptions1(PyObject *);
 
 /* In exceptions.c */
 
+PyAPI_FUNC(int) _PyUnicodeError_GetParams(
+    PyObject *self,
+    PyObject **obj, Py_ssize_t *objlen,
+    Py_ssize_t *start, Py_ssize_t *end,
+    int as_bytes);
+
 PyAPI_FUNC(PyObject*) PyUnstable_Exc_PrepReraiseStar(
      PyObject *orig,
      PyObject *excs);