'once_registry'. This is bad as the warnings module had variables named
'defaultaction' and 'onceregistry' which are what people should be looking at
(technically those variables shouldn't be mucked with as they are undocumented,
but we all know better than to believe that isn't happening). So the variables
from _warnings have been renamed to come off as private and to avoid confusion
over what variable should be used.

Closes issue #9766. Thanks to Antoine Pitrou for the discovery.