contrib/wix/defines.wxi

pycompat: when setting attrs, ensure we use sysstr

The custom module importer was making these bytes, so when we poked
values into self.__dict__ we had bytes instead of unicode on py3 and
it didn't work.

<Include>

  <?if $(var.Platform) = "x64" ?>
    <?define IsX64 = yes ?>
  <?else?>
    <?define IsX64 = no ?>
  <?endif?>

</Include>