Migration of sale_automatic_workflow to 8.0

# Migration of sale_automatic_workflow to 8.0

* Added some tests
* Removed the dependency on `sale_payment_method`: we should be able to use automatic workflows even if we don't use the sale payment methods. We will need to reimplement the few features I removed in a third "link" module between `sale_automatic_workflow` and `sale_payment_method`.
* Use new API whenever possible
* Updated `pot`
Is it the only method not ported to api 8.0? Any reason for that?
An `@api.model` should work here to make the module fully ported.