Deprecate None values for Date, Datetime and Time trait types

[mdickinson]

This PR builds on #1441 to also deprecate acceptance of None values for Date traits. While I think this is the right way to go, this does introduce a discrepancy with the Datetime and Time classes, where None is still accepted by default with no deprecation warning issued.

Update: the PR has been extended to cover Datetime and Time classes; this required realising the Datetime and Time trait types (so that they're no longer instances of BaseInstance).

Closes #1438

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference) (relevant docstrings updated)
• Update User manual (docs/source/traits_user_manual) N/A
• Update type annotation hints in traits-stubs

[mdickinson]

Drive-by copypasta fix, unrelated to the main purpose of the PR

[rahulporuri]

LGTM with one minor question. We already have the issue regd documenting Date, Datetime and Time so there's no documentation changes that need to be made in this PR.

Just to confirm, can there be any issues rewriting the traits as subclasses of TraitType instead of using BaseInstance?

Finally, how do we plan to track the work needed for traits 7.0.0? Do we want to create an issue now/a milestone?

[mdickinson]

Just to confirm, can there be any issues rewriting the traits as subclasses of TraitType instead of using BaseInstance?

Yes. There are potential behaviour changes here, but mostly in a good direction: that is, treating an instance of TraitType as though it were a subclass of TraitType is fraught with issues.

For example, if anyone is "instantiating" Datetime or Time in their code and making use of the particular keyword arguments that BaseInstance understands, their code will change behaviour.

But I checked existing code (to the extent that that code was available to me) for usage patterns, and I would expect the vast majority of existing code to be unaffected by this change (beyond the intentional part of the change - the new DeprecationWarning).

[mdickinson]

Finally, how do we plan to track the work needed for traits 7.0.0? Do we want to create an issue now/a milestone?

The milestone already exists, and has done for some time. I already have an issue in that milestone as a reminder to review deprecations.

[mdickinson]

I already have an issue in that milestone as a reminder to review deprecations.

#1268