tools: python3 compat for inspector code generator

The code generator takes a dict and turns it into a namedtuple. The dict
contains the key "async", which is a keyword in python 3.7, and rejected
by the namedtuple constructor. Rename it to "async_" to avoid the clash.

Fixes: https://github.com/nodejs/node/issues/29326

PR-URL: https://github.com/nodejs/node/pull/29340


Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>