title: How to add custom css class to django-filters field
body: <p>I tried to make a way that worked with forms, but here the class did not apply</p>
  
  <pre><code>class TrainerFilter(django_filters.FilterSet):
      price_lt = django_filters.NumberFilter(field_name="prise", lookup_expr='lt')
      class Meta:
          model = Profile
          fields = ['direction', 'group', 'child', 'price_lt']
          widgets = {
              'direction': SelectMultiple(attrs={'class': 'custom-select'}),     
          }
  </code></pre>