gyp: futurize imput.py to prepare for Python 3

PR-URL: https://github.com/nodejs/node/pull/29140


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>