[3.7] bpo-24214: Fixed the UTF-8 and UTF-16 incremental decoders. (GH-14304) (GH...

[3.7] bpo-24214: Fixed the UTF-8 and UTF-16 incremental decoders. (GH-14304) (GH-14369)

* bpo-24214: Fixed the UTF-8 and UTF-16 incremental decoders. (GH-14304)

* The UTF-8 incremental decoders fails now fast if encounter
  a sequence that can't be handled by the error handler.
* The UTF-16 incremental decoders with the surrogatepass error
  handler decodes now a lone low surrogate with final=False.
(cherry picked from commit 894263ba80af4b7733c2df95b527e96953922656)

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