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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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']