[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 ce1b747ff6)

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

Python/codecs.c

@@ -1087,7 +1087,7 @@ get_standard_encoding(const char *encoding, int *bytelength)
             }
         }
     }
-    else if (strcmp(encoding, "CP_UTF8") == 0) {
+    else if (strcmp(encoding, "cp65001") == 0) {
         *bytelength = 3;
         return ENC_UTF8;
     }