support library listeners

[DetachHead]

ROBOT_LIBRARY_LISTENERS variable doesn't work in python tests since modules are not imported by robot's module import machinery

[DetachHead]

maybe pytest hooks are better suited for this? maybe we should make a listener that exports pytest hooks for every listener event, then you can implement those hooks only in child conftest.py files

the downside is that hook functions are more annoying to use and less typesafe than listeners, since they dont use classes (pytest-dev/pytest#11300)