I am trying to set a bootstrap-select class to my choice field. However, I am getting a 'TypeError: **init**() got an unexpected keyword argument 'attrs''.

```
class constraintListForm1(forms.Form):

    region = forms.ChoiceField(choices=REGION, required=True)   
    operator = forms.ChoiceField(choices=OPERATOR, required=True )

    class META:
        model = constraintListModel1  

        widgets = {
            'region': forms.ChoiceField(attrs={'class' : 'bootstrap-select'}),

            }

```