docs: def clean_username(self):
          """
          Validate that the username is unique and not listed 
          in ``defaults.ACCOUNTS_FORBIDDEN_USERNAMES`` list.
          
          """
          try: 
              user = get_user_model().objects.get(username=self.cleaned_data["username"])
          except get_user_model().DoesNotExist: 
              pass
          else: 
              raise forms.ValidationError(
                              self.error_messages['duplicate_username'])
  
          if self.cleaned_data['username'].lower() \
              in defaults.ACCOUNTS_FORBIDDEN_USERNAMES:
              raise forms.ValidationError(_(u'This username is not allowed.'))
          return self.cleaned_data['username']