fix(stock): skip returns when recalculating item last purchase rate

(cherry picked from commit e27989f201)

erpnext/stock/doctype/item/item.py

@@ -1371,6 +1371,7 @@ def get_purchase_voucher_details(doctype, item_code, document_name=None):
 		query = query.where(parent_doc.name != document_name)
 
 	if doctype in ("Purchase Receipt", "Purchase Invoice"):
+		query = query.where(parent_doc.is_return == 0)
 		query = query.select(parent_doc.posting_date, parent_doc.posting_time)
 		query = query.orderby(
 			parent_doc.posting_date, parent_doc.posting_time, parent_doc.name, order=Order.desc