States open, paid, in_payment are removed.

There is now a new invoice_payment_state field in Odoo 13, defined as:

invoice_payment_state = fields.Selection(selection=[
('not_paid', 'Not Paid'),
('in_payment', 'In Payment'),
('paid', 'Paid')],
string='Payment', store=True, readonly=True, copy=False, tracking=True,
compute='_compute_amount')

https://apps.odoo.com/apps/modules/13.0/sale_invoice_status/

Looks like the Open state has been removed. Now there are only 3 states - Draft, Posted and Cancelled. 

EDIT: The Invoice now uses the account.move model instead of the account.account model. Extensive changes have been made to the Accounting/Invoicing functionality in v13.