tools: replace deprecated iteritems() for items()

This is a Built-In change in Python 3 used by compile_commands_json.py
to generate compile_commands_json.json. Node.js dropped Python 2 support
in this script, `items()` works as expected replacement.

PR-URL: https://github.com/nodejs/node/pull/31528


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>