[FIX] fix product.template related fields

make product.template.product_variant_id a stored computed field to
avoid the following error when setting the print_category_id or to_print
related fields: "non-stored field product.template.product_variant_id
cannot be searched."

product_print_category/models/product_template.py

@@ -9,6 +9,9 @@ class ProductTemplate(models.Model):
     _name = "product.template"
     _inherit = ["product.template", "product.print.category.mixin"]
 
+    # store this field (required for the related fields below)
+    product_variant_id = fields.Many2one(store=True)
+
     print_category_id = fields.Many2one(
         related="product_variant_id.print_category_id", readonly=False
     )