bpo-24214: Fixed the UTF-8 incremental decoder. (GH-12603) (GH-12627)

The bug occurred when the encoded surrogate character is passed
to the incremental decoder in two chunks.
(cherry picked from commit 7a465cb5ee7e298cae626ace1fc3e7d97df79f2e)

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