Add explicit allow_none in Datetime traits.

[corranwebster]

Add allow_none to Datetime bounds in DatetimeEditor and to other uses of Datetime in tests and examples.

This fixes a deprecation warning with recent traits.

Fixes #1963.

Checklist

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[mdickinson]

LGTM

[mdickinson]

Off-topic for this PR, but the DateEditorDemo name is surprising, giving that we're editing a Datetime trait rather than a Date trait.

[corranwebster]

Likely either copy/paste error or reflecting the fact that there was no Datetime trait until comparatively recently.

[mdickinson]

I guess we don't need corresponding changes in the DatetimeEditor editor factory class (traitsui/editors/datetime_editor.py), because it's unlikely that anyone would be deliberately setting a minimum_datetime or maximum_datetime of None there.

[corranwebster]

I guess we don't need corresponding changes in the DatetimeEditor editor factory class (traitsui/editors/datetime_editor.py), because it's unlikely that anyone would be deliberately setting a minimum_datetime or maximum_datetime of None there.

Hmmmm... it feels like those probably should as well? It's possible that they were missed when grepping for datetime usages.

[mdickinson]

Hmmmm... it feels like those probably should as well?

Sure, it certainly wouldn't harm.