docs: def clean_username(self):
          """
          Validate that the username is alphanumeric and is not already in use.
          Also validates that the username is not listed in
          ACCOUNTS_FORBIDDEN_USERNAMES list.
          """
          try:
              get_user_model().objects.get(
                  username__iexact=self.cleaned_data['username'])
          except get_user_model().DoesNotExist:
              pass
          else:
              raise forms.ValidationError(_('This username is already taken.'))
          if self.cleaned_data['username'].lower() in accounts_settings.ACCOUNTS_FORBIDDEN_USERNAMES:
              raise forms.ValidationError(_('This username is not allowed.'))
          return self.cleaned_data['username']