Update Changelog

Author: mllemango

CHANGELOG

@@ -1,5 +1,7 @@
 == Unreleased
 
+- Update API version with 2021-07 release, remove API version 2020-01 ([#521](https://github.com/Shopify/shopify_python_api/pull/521))
+
 == Version 8.4.1
 - Bug fix: `sanitize_shop_domain` now returns `None` rather than `'none.myshopify.com'` if no `shop_domain` arg is passed in ([#499](https://github.com/Shopify/shopify_python_api/pull/499))