Make Plone object undeletable but still work with iterate / stagingbehavior

configure.zcml

<configure xmlns="http://namespaces.zope.org/zope">
  <subscriber
      for=".interfaces.IMyType
           OFS.interfaces.IObjectWillBeRemovedEvent"
      handler=".events.event_handler"
      />
</configure>

events.py

from AccessControl import Unauthorized
try:
    from plone.app.iterate.interfaces import CheckinException
    from plone.app.iterate.interfaces import IObjectCopier
    HAS_ITERATE = True
except ImportError:
    # No iterate available
    HAS_ITERATE = False
from zope.component import queryAdapter


def event_handler(object, event):
    if HAS_ITERATE:
        copier = queryAdapter(object, IObjectCopier)
        baseline = None
        try:
            baseline = copier._getBaseline()
        except CheckinException:
            pass
        
        if baseline is not None:
            # This is a working copy created by plone.app.iterate /
            # plone.app.stagingbehavior. We need to be able to move and delete
            # these.
            return
    
    raise Unauthorized