`BaseStage` instance has no `_attrs_to_hash` member...

[thehrh]

... even though the hash property and the include_attrs_for_hashes method require it. This means that calling hash on any PiStage which doesn't define the member itself will fail (which is how I discovered it in the first place). It seems it didn't get migrated from Stage (looks like someone forgot to run pylint :P)