Ratchet down Python warnings in tox

Where possible, have tox envs treat any Python warning as an error.
This is not currently possible for any envs which invoke certain PBR
functionality, since it has a couple of outstanding fixes under
review to make it cease calling into deprecated code on newer Python
releases. Eventually this should be set in the global testenv when
possible, and only rolled back in case of dependency regressions or
new errors introduced by future interpreter versions.