Add parâmetro no forms | ul_attrs

forms.py

from django import forms
from .widgets import MyCheckboxSelectMultiple

class Form(forms.Form):
    ability = forms.ChoiceField(
        widget= MyCheckboxSelectMultiple(ul_attrs={"class":"fancybox"}), 
        choices = SKILLS, 
        required=False
        )

widgets.py

from django import forms    
from django.forms.util import flatatt
from django.utils.safestring import mark_safe

class MyCheckboxSelectMultiple(forms.CheckboxSelectMultiple):
def __init__(self, attrs=None, ul_attrs=None):
    self.ul_attrs = ul_attrs
    super(MyCheckboxSelectMultiple, self).__init__(attrs)

def render(self, name, value, attrs=None, choices=()):
    html = super(MyCheckboxSelectMultiple, self).render(name, value, attrs, choices)
    final_attrs = self.build_attrs(self.ul_attrs)
    return mark_safe(html.replace('<ul>','<ul%s>' % flatatt(final_attrs)))