tools: use `is None` consistently in Python

We use `is None` instead of `== None` everywhere (which mostly just
means test.py) except in one place in genv8constants.py. Switch to `is
None` in genv8constants.py. This is slightly more efficient, although I
can't imagine that makes a measurable difference here.

PR-URL: https://github.com/nodejs/node/pull/36606


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>