Add PYTHONUTF8 to commandline usage. (GH-17587)

Co-Authored-By: Victor Stinner <vstinner@python.org>

Python/initconfig.c

@@ -80,6 +80,7 @@ static const char usage_5[] =
 "PYTHONHOME   : alternate <prefix> directory (or <prefix>%lc<exec_prefix>).\n"
 "               The default module search path uses %s.\n"
 "PYTHONCASEOK : ignore case in 'import' statements (Windows).\n"
+"PYTHONUTF8: if set to 1, enable the UTF-8 mode.\n"
 "PYTHONIOENCODING: Encoding[:errors] used for stdin/stdout/stderr.\n"
 "PYTHONFAULTHANDLER: dump the Python traceback on fatal errors.\n";
 static const char usage_6[] =