title: Use Selection Field in Another Model in Odoo 13
body: <p>I'm totally a newbie here who love Odoo so much :)</p>
  <p>I got stuck with this problem for a few days in Odoo CE 13.</p>
  <p>I want my SHIPPING TYPE field which I made in <em><strong>sale.order</strong></em> can be used in <em><strong>purchase.order</strong></em> as well.</p>
  <p>So I did this in <em><strong>shipping_type.py</strong></em></p>
  <pre><code>  # -*- coding: utf-8 -*-
      from odoo import models, fields
      
      class SaleOrder(models.Model):
          _inherit = 'sale.order'
      
          shipping_selection = [
                      ('type1', 'Instant'),
                      ('type2', 'Same Day'),
                      ('type3', 'JNE'),
                      ('type4', 'Tokopedia'),
                      ('type5', 'Pick Up'),
                      ('type6', 'AnterAja-Reguler'),
                      ('type7', 'J&amp;T-Reguler'),
                      ('type8', 'Ninja-Reguler'),
      
                       ]
      
          shipping_type = fields.Selection(shipping_selection,'Shipping Type',)
  </code></pre>
  <p>Then, I was trying to make <em><strong>purchase_shipping_type.py</strong></em> in another addons, and created this:</p>
  <pre><code># -*- coding: utf-8 -*-
  from odoo import models, fields
  
  class SaleOrder(models.Model):
      _inherit = ['sale.order']
  
      shipping_selection = [
                  ('type1', 'Instant'),
                  ('type2', 'Same Day'),
                  ('type3', 'JNE'),
                  ('type4', 'Tokopedia'),
                  ('type5', 'Pick Up'),
                  ('type6', 'AnterAja-Reguler'),
                  ('type7', 'J&amp;T-Reguler'),
                  ('type8', 'Ninja-Reguler'),
  
                   ]
  
      shipping_type = fields.Selection(shipping_selection,'Shipping Type',)
  
  class PurchaseOrder(models.Model):
      _inherit = ['purchase.order']
  
     shipping_type_purchase = fields.Selection(string='Shipping Type',related=shipping_type.shipping_type_purhase, readonly=True)
  </code></pre>
  <p>This error showed up:</p>
  <pre><code>Aug 21 06:29:03 kama-odoo-server odoo13[24202]: NameError: name 'shipping_type' is not defined - - -
  </code></pre>
  <p>How to set up this properly? I really hope someone can help me :) Thanks!</p>