Add new price type attributes (#106)

* add new price_type and price_type_label attributes

* Use PriceType model from SDK

Co-authored-by: julianfrenzel <26822719+b0lle@users.noreply.github.com>

Author: sergioteula

amazon_paapi/models/item_result.py

@@ -45,7 +45,7 @@ class ApiPrice:
     currency: str
     price_per_unit: float
     display_amount: str
-    price_type: str
+    price_type: models.PriceType
     price_type_label: str