D4432: py3: do .decode(), .encode() dance in runtests.py

[Yuya Nishihara]

>          def toggletrace(cmd):
> -            quoted = shellquote(cmd.strip()).replace(b'\\', b'\\\\')
> +            cmd = cmd.decode('ascii')
> +            quoted = shellquote(cmd.strip()).encode('ascii')
> +            quoted = quoted.replace(b'\\', b'\\\\')

Perhaps _bytespath/_strpath can be used to avoid possible side effect on
Python 2.