I'm hesitant to put the *true* default filters in sys.warnoptions, as folks use "bool(sys.warnoptions)" as a check for "Were any warning options set via the environment or the command line?", and we don't want to invalidate that use case.

However, I'm definitely a fan of having the warnings module *only* look at sys.warnoptions, and requiring *all* command line options to route their filter settings through that channel.

https://bugs.python.org/issue32230 adjusts "-X dev" to work that way, and I'm suggesting we do the same here for "-b".

That way, we can fully define the relative ordering of PYTHONWARNINGS, "-X dev", "-W", and "-b" inside pymain_add_warnings_options (and only the true default filters will be appended by the warnings module itself).