gh-58124: Avoid CP_UTF8 in UnicodeDecodeError (#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.

Python/codecs.c

@@ -1204,7 +1204,7 @@ get_standard_encoding_impl(const char *encoding, int *bytelength)
             }
         }
     }
-    else if (strcmp(encoding, "CP_UTF8") == 0) {
+    else if (strcmp(encoding, "cp65001") == 0) {
         *bytelength = 3;
         return ENC_UTF8;
     }