[FIX] l10n_es_aeat_sii_oca: Change registration key on FP change

Steps to reproduce:
- Have partner 1 with fiscal position FP1 with registration key 01.
- Have partner 2 with fiscal position FP2 with registration key 02.
- Create customer invoice with partner 1 as customer.
- Select partner 2 as delivery address.

Expected result:
Fiscal position is changed to FP2 and the registration key is changed
to 02.

Current behavior:
The fiscal position is changed, but not the registration key.

That's because the recursive onchanges are explicitly disabled in
account move object:

https://github.com/odoo/odoo/blob/2f817a7b36cc7e5ab235829eecd61a1d71ce546e/addons/account/models/account_move.py#L1190

We workaround this limitation converting the registration key into a
computed writable field that is computed even if the recursive onchange
is disabled.

l10n_es_aeat_sii_oss/__manifest__.py

@@ -3,7 +3,7 @@
 
 {
     "name": "Suministro Inmediato de Información en el IVA: OSS",
-    "version": "15.0.1.0.0",
+    "version": "15.0.1.0.1",
     "category": "Accounting & Finance",
     "website": "https://github.com/OCA/l10n-spain",
     "author": "FactorLibre, Tecnativa, Odoo Community Association (OCA)",

l10n_es_aeat_sii_oss/tests/test_l10n_es_aeat_sii.py

@@ -51,4 +51,3 @@ def test_invoice_sii_oss(self):
         res = invoice._get_sii_invoice_dict()
         res_issue = res["FacturaExpedida"]
         self.assertEqual(res_issue["ImporteTotal"], 100)
-        self.assertEqual(res_issue["ClaveRegimenEspecialOTrascendencia"], "17")