gyp: fix build with python 2.6

Fixes: https://github.com/joyent/node/issues/6859
PR-URL: https://github.com/iojs/io.js/pull/1325


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>