Function PyUnicode_CopyCharacters 

pub unsafe extern "C" fn PyUnicode_CopyCharacters(
    to: *mut PyObject,
    to_start: isize,
    from: *mut PyObject,
    from_start: isize,
    how_many: isize,
) -> isize

Available on neither Py_LIMITED_API nor RustPython and non-PyPy only.