py3: switch from iteritems() to items() in the topics extension The only remaining iteritems() call is on branchmap. That will be dealt with in the next patch.

py3: filter() now returns a generator, so wrap when we need a list

py3: don't depend on map() iterating over its input map(some_generator()) in py2 returns a list, while in py3 it returns a generator, so the passed-in generator won't be called unless the returned one is.