[3.13] gh-58124: Avoid CP_UTF8 in UnicodeDecodeError (GH-137415) (#137461)

gh-58124: Avoid CP_UTF8 in UnicodeDecodeError (GH-137415)

Fix name of the Python encoding in Unicode errors of the code page
codec: use "cp65000" and "cp65001" instead of "CP_UTF7" and "CP_UTF8"
which are not valid Python code names.
(cherry picked from commit ce1b747ff68754635b7b12870dfc527184ee3b39)

Co-authored-by: Victor Stinner <vstinner@python.org>