[3.11] gh-91810: ElementTree: Use text file's encoding by default in XML declaration (GH-91903) (GH-92663)

ElementTree method write() and function tostring() now use the text file's
encoding ("UTF-8" if not available) instead of locale encoding in XML
declaration when encoding="unicode" is specified.
(cherry picked from commit 707839b0fe02ba2c891a40f40e7a869d84c2c9c5)


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

Automerge-Triggered-By: GH:serhiy-storchaka