gyp: futurize imput.py to prepare for Python 3

PR-URL: #29140
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: Sam Roberts <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: Jiawen Geng <[email protected]>
Reviewed-By: Ben Noordhuis <[email protected]>