[3.13] gh-148821: Add more strict tests for XML encodings (GH-149765) (GH-149772)

Exclude encodings like 'utf-8-sig', 'iso2022-jp' and 'hz' from the list of
supported encodings.
(cherry picked from commit fa2afa64d9467fb7362672ed603d29d8e246d240)

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