gyp: make StringIO work in ninja.py

Allow both Python 2 and 3 to access StringIO.

This fixes `./configure --ninja`, which was broken by
https://github.com/nodejs/node/pull/29371.

See: https://github.com/nodejs/node/pull/29371#issuecomment-527331969

PR-URL: https://github.com/nodejs/node/pull/29414


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>