- Added a new keyword argument ``once=True`` to :func:`.event.listen`
and :func:`.event.listens_for`.  This is a convenience feature which
will wrap the given listener such that it is only invoked once.
- A warning is emitted if the :meth:`.MapperEvents.before_configured`
or :meth:`.MapperEvents.after_configured` events are applied to a
specific mapper or mapped class, as the events are only invoked
for the :class:`.Mapper` target at the general level.