An addition to Add python warning if install a pack that only supports python 2 #5037 PR and Python 2 Deprecation Game Plan, apart of warning for pack install, we might need to also warn users that's running StackStorm in py2 environment.

I'd suggest to show and log that warning for each st2 service during its initial startup, but open to other ideas.
Maybe also show for every st2ctl execution?