error: "Collector" has no attribute "data"

[benwhalley]

Mypy complains about this code:

collector = Collector(using="default")
collector.collect([self])
children = list(itertools.chain(*[v for k, v in collector.data.items()]))

saying error: "Collector" has no attribute "data"

I'm not sure what the most general type should be, but collectors do have a data attribute.