Datetime widget fixes

[vidartf]

• Adds a missing export
• Fixes AttributeError raised by DatetimePicker  #3500 (I'm still interested in figuring out what is going wrong there, but this fix should be ok).

[github-actions]

👈 Launch a binder notebook on branch vidartf/ipywidgets/datetime-fix

[jasongrout]

Thanks. Can you add documentation in the Widget List page for NaiveDateTime?

I was looking at the api, and I think arguably the default date/time picker probably should be the naive one, just like the generic python date/time is naive by default. I think the datetime picker is most often used for filtering historical data, and I think generally users want to pick date/times according to the source data's convention. In other words, if I'm in timezone +5, and I select data from midnight of a certain date to midnight of another date, I think it is most likely I mean midnight of the data's date, not midnight in whatever timezone I happen to be in (i.e., I most likely am wanting the naive date/time). However, we've already shipped 8.0, so perhaps that ship has sailed, and we should just really clearly document the differences between the two, and note when a person might want to use one vs another.

[jasongrout-db]

However, we've already shipped 8.0, so perhaps that ship has sailed

Also, I didn't realize this matches the api in ipydatetime (of course, since you authored both :), so in a sense it is backwards compatible with this api. So yeah, I think just adding more explanation about the NaiveDateTimePicker in the docs, and when you would want to use it vs the DateTimePicker, would be great.

[vidartf]

I ported the docs over from ipydatetime (simplified the code).

[jasongrout]

Thanks!