The `-X dev` option is currently integrated directly with the warnings module, rather than working indirectly through `sys.warnoptions`.

This means that any third party code that currently checks sys.warnoptions will need to be updated to check sys.flags.dev_mode as well.

Rather than doing that, I propose that we instead change the way dev mode works to *literally* be equivalent to `-Wdefault`, and remove the direct integration with the warnings machinery.

(PR for that coming shortly)