Add a CI job that errors on warnings

[bhazelton]

This would help make it obvious when new deprecation warnings start happening, which will allow you to adapt code before it breaks.

You can make pytest error on all warnings with -W error or just on deprecation warnings with -W "error::DeprecationWarning". We have a job like this on all RASG repos and it's super helpful. We don't require that it passes to merge a PR but we usually notice and fix the problem as soon as the job starts erroring.