[3.10] bpo-45467: Fix IncrementalDecoder and StreamReader in the "raw-unicode-escape" codec (GH-28944) (GH-28952)

They support now splitting escape sequences between input chunks.

Add the third parameter "final" in codecs.raw_unicode_escape_decode().
It is True by default to match the former behavior.
(cherry picked from commit 39aa98346d5dd8ac591a7cafb467af21c53f1e5d)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>