Allow Date trait type to reject datetime instances

[mdickinson]

This PR makes it possible to specify that a Date trait should not accept datetime instances, which is usually the behaviour that the user expects/wants.

• Adds an allow_datetime parameter to Date. It defaults to True, for backwards compatibility.
• To make this possible, converts Date to a proper TraitType subclass.

Closes some but not all of #398. I'd like to leave that issue open for the question of considering changing the allow_datetime default under a deprecation warning.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• [ ] Update User manual (docs/source/traits_user_manual) N/A - no user documentation for Date
• Update type annotation hints in traits-stubs

[mdickinson]

@kitchoi Would the change in this PR meet your needs?

[kitchoi]

LGTM. Thank you.

[kitchoi]

The other trait types don't seem to need this Optional bit. I don't think I understand why this is needed here.

[mdickinson]

It's there because None is a valid value for this trait. The other trait types (Datetime and Time) should have this, I think, else mypy will complain about the assignment of None.

[mdickinson]

(And arguably, None should not be a valid trait value here. That's where I'd like to get to, but we can't do it immediately because of backwards compatibility concerns.)

[mdickinson]

Ah, it is already there for Datetime and Time: the possibility of None is there in the _BaseClass generic definition. We've switched from using _BaseInstance as a base here to something more specific (and more accurate).