SDI validation fails for Italian PA

[libremente]

Hi!
Italian Public administrations have a 6 chars long SDI (see here for example https://www.fattura.it/guide/codice-sdi/8/).
Right now the check fails when inserting less than 7 chars (see https://github.com/pythonitalia/pycon/blob/main/backend/billing/validation.py#L23-L24).

I guess this check should be relaxed or there should be a flag for enabling the "PA" case.
Thanks

[patrick91]

Thanks for the report! I've pushed a PR here #4406