after all that, use pytest warnings plugin

after all that, use pytest warnings plugin

The warnings plugin lets us set the filters up
in the config, and as our filter requirements are now
simple we can just set this up.

additionally pytest now recommends pyproject.toml, since
we fully include this now, let's move it there.

the pytest logging plugin seems to not be any problem either
at the moment, so re-enable that.  if it becomes apparent
whatever the problem was (which was probably that it was just
surprising, or something) we can disable it again and comment
what the reason was.

Change-Id: Ia9715533b01f72aa5fdcf6a27ce75b76f829fa43