fix py3 version of calibredb show_metadata

src/calibre/db/cli/cmd_show_metadata.py

@@ -49,8 +49,9 @@ def main(opts, args, dbctx):
     if mi is None:
         raise SystemExit('Id #%d is not present in database.' % id)
     if opts.as_opf:
+        stdout = getattr(sys.stdout, 'buffer', sys.stdout)
         mi = OPFCreator(getcwd(), mi)
-        mi.render(sys.stdout)
+        mi.render(stdout)
     else:
         prints(unicode_type(mi))