Procuret

- `PaymentSeries.payment_method` is now an `Optional[PaymentMethodHeadline]`, it was formerly of type `PaymentMethodHeadline`. In rare cases, a `PaymentSeries` may not have an associated payment method, and this changes allows Procuret Python to correctly decode such cases.
- Fixes a bug causing `TermRateGroup` retrieval to fail due to attempting to decode properties that that API no longer sends.

- `SecondFactorCode`, now required for all `Session` creation. `Session.create...` methods updated accordingly.
- `SaleNomenclature` enumeration
- New properties for `InstalmentLink`
- `.allow_edit: bool`
- `.sale_name: SaleNomenclature`
- `.denomination_id: int`
- `.denomination: Currency`

0.0.5 introduces the `InstalmentLinkOpen` class, which may be used to record an instance of an `InstalmentLink` being opened. Instances of opening are recorded in the new `InstalmentLink.opens` property.