Add simple way to use SplitDateTimeField on ModelForms

[David Seddon]

If you specify a SplitDateTimeWidget as the custom widget for a DateTimeField on a ModelForm, you receive this deprecation warning:

RemovedInDjango19Warning: Using SplitDateTimeWidget with DateTimeField is deprecated. Use SplitDateTimeField instead.

My understanding is that the only way to do this for a ModelForm based on a model with a DateTimeField, is to subclass models.DateTimeField like this: ​https://gist.github.com/nacady/8dd5cab2f749d1589f1e.

Perhaps we should provide a simpler way of using the SplitDateTimeField with ModelForms?

[Tim Graham]

The ModelForm.Meta.field_classes feature added in Django 1.9 should allow this (#24295).

[David Seddon]

Oh, great feature - yes, this will do the trick.