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`
> You can use @api.v7 and @api.v8 trick. There are some samples in core code.

I know @api.v7 and @api.v8, but IMO they should be used at the first declaration of the method, not in an `_inherit`. What if another module declare another signature?