scripts: gen_kobject_list.py: Simplify test with 'in'

Getting slightly subjective, but fixes this pylint warning:

    scripts/gen_kobject_list.py:308:11: R1714: Consider merging these
    comparisons with "in" to "kobj in ('device',
    '_k_thread_stack_element')" (consider-using-in)

Use a set literal instead of a tuple literal, as recent Python 3
versions optimize set literals with constant keys nicely.

Getting rid of pylint warnings for a CI check. I could disable any
controversial ones (it's already a list of warnings to enable anyway).

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>
diff --git a/scripts/gen_kobject_list.py b/scripts/gen_kobject_list.py
index 22fd065..0d82652 100755
--- a/scripts/gen_kobject_list.py
+++ b/scripts/gen_kobject_list.py
@@ -305,7 +305,7 @@
         dep, _ = obj_info
         # device handled by default case. Stacks are not currently handled,
         # if they eventually are it will be a special case.
-        if kobj == "device" or kobj == "_k_thread_stack_element":
+        if kobj in {"device", "_k_thread_stack_element"}:
             continue
 
         if dep: