I like the libcst approach, and the ones I checked seem to all be correct.

Technically speaking, this has little to do with static typing, but can be very helpful for IDE introspection, which is also one of the main advantages of annotations, so I suppose it's fine to keep the TYP: label.

For most defaults I can see that they can be useful. But in some cases, like out=None, I'm not if the defaults are actually helpful. Because without annotations, just having out=None doesn't give you any additional information about how it can be used. It could even be a bit confusing this way if some parameters use =None and others =np._NoValue (i.e. =...), especially if you consider that the documentation of _NoValue often incorrectly says it defaults to None. That could be confusing because it appears to be inconsistent.

Anyway, it probably doesn't matter much, so I'm fine with keeping those =None. Removing them now would mean we'd have to add them back in again once we add the annotations.