D3464: scmutil: clean up bytes/string cache decorator mess on Python 3 again

[Yuya Nishihara]

Fixed the last AttributeError to raise the "s" name.

On Tue, 8 May 2018 04:26:35 +0000, durin42 (Augie Fackler) wrote:
>      def __delete__(self, obj):
>          try:
> -            del obj.__dict__[self.name]
> +            del obj.__dict__[self.sname]
>          except KeyError:
>              raise AttributeError(self.name)