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: #36606
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

tools/genv8constants.py

@@ -95,7 +95,7 @@ def out_define():
       curr_octet += 1
 
   match = pattern.match(line)
-  if match == None:
+  if match is None:
     continue
 
   # Print previous symbol