I was about to argue for your initial version! I would say that utcnow() is marginally acceptable (though only on backwards-compatibility grounds), but only if the conversion from a naive time to aware time raises an error if done without an explicit timezone specification. It is the combination keeping utcnow() as-was (or, more generally, the concept of naive time at all), while introducing a default for the conversion, that turns it into a subtle trap.

Unfortunately, now that the default has been introduced, there seems no way to get to get out of this situation while preserving backwards-compatibility. The lesson here, perhaps, is to not make it easier to use that which should have been deprecated.

Thanks for your clear and to-the-point article!