Impacted versions: 9.0

Steps to reproduce:

* Create an MTO product with 
  * Product Type: Stockable Product
  * Routes: "Buy" and "Make to Order"
* Update Qty on Hand to any quantity (i.e, 2).
* Create a sales order with this product and a number <= to the one in the step before.
* Confirm the sales order.

Current behavior:

The created delivery picking appears in state "Reserved" and the current quantity on stock has been reserved, allowing you to transfer that stock.

Expected behavior:

The created delivery picking should be in "Waiting another operation", or at least in "Waiting availability" until the purchase order is confirmed (if you have defined a supplier for the product and the procurement creates the purchase order), which seems to be the current behavior when there's no stock available (is this also correct, as it allows to manually reserve?).
addons/stock/stock.py
""" Changes the state to assigned.
        @return: True
        """
        res = self.write(cr, uid, ids, {'state': 'assigned'}, context=context)
        self.check_recompute_pack_op(cr, uid, ids, context=context)
        return res
    def check_tracking(self, cr, uid, move, ops, context=None):
        """ Checks if serial number is assigned to stock move or not and raise an error if it had to.
        """
        if move.picking_id and (move.picking_id.picking_type_id.use_existing_lots or move.picking_id.picking_type_id.use_create_lots) and \
            move.product_id.tracking != 'none':
            if not (move.restrict_lot_id or (ops and (ops.product_id and ops.pack_lot_ids)) or (ops and not ops.product_id)):
                raise UserError(_('You need to provide a Lot/Serial Number for product %s') % move.product_id.name)
    def check_recompute_pack_op(self, cr, uid, ids, context=None):
        pickings = list(set([x.picking_id for x in self.browse(cr, uid, ids, context=context) if x.picking_id]))
        pickings_partial = []
        pickings_write = []
        pick_obj = self.pool['stock.picking']
        for pick in pickings:
            if pick.state in ('waiting', 'confirmed'): 
                continue
            
            if not any([x.qty_done > 0 for x in pick.pack_operation_ids]):
                pickings_partial.append(pick.id)
            else:
                pickings_write.append(pick.id)
        if pickings_partial: