fix: fallback to 'price' in ecommerce api loader

The Django Oscar upgrade of ecommerce changed the
item's price field from `price_excl_tax` to just `price`
causing the EcommerceApi data loader to fail.

This commit checks for the `price_excl_tax` key and
falls back to the 'price' value.

Ref: openedx/ecommerce#4050

course_discovery/apps/course_metadata/data_loaders/api.py

@@ -572,7 +572,10 @@ def update_seats(self, body):
     def update_seat(self, course_run, product_body):
         stock_record = product_body['stockrecords'][0]
         currency_code = stock_record['price_currency']
-        price = Decimal(stock_record['price_excl_tax'])
+        if 'price_excl_tax' in stock_record:
+            price = Decimal(stock_record['price_excl_tax'])
+        else:
+            price = Decimal(stock_record['price'])
         sku = stock_record['partner_sku']
 
         # For more context see ADR docs/decisions/0025-dont-sync-mobile-skus-on-discovery.rst