Message 307719 - Python tracker

I filed https://bugs.python.org/issue32231 to cover having -bb always override other warnings settings when it comes to handling BytesWarning.

Writing that issue made me realise another quirk with `-X dev` though: as a command line option, it should really take precedence over PYTHONWARNINGS, while remain subordinate to other explicit -W options.

I've adjusted the PR to implement that change, but haven't added a test case that ensures the behaviour persists.