Update to 2021-10 release

Author: andyw8

CHANGELOG

@@ -1,6 +1,7 @@
 == Unreleased
 
 - Drop Python 2 support ([#549](https://github.com/Shopify/shopify_python_api/pull/549))
+- Update API version with 2021-10 release, remove API version 2020-04 ([#548](https://github.com/Shopify/shopify_python_api/pull/548))
 
 == Version 8.4.2
 - Update API version with 2021-07 release, remove API version 2020-01 ([#521](https://github.com/Shopify/shopify_python_api/pull/521))

shopify/api_version.py

@@ -27,12 +27,12 @@ def define_version(cls, version):
     @classmethod
     def define_known_versions(cls):
         cls.define_version(Unstable())
-        cls.define_version(Release("2020-04"))
         cls.define_version(Release("2020-07"))
         cls.define_version(Release("2020-10"))
         cls.define_version(Release("2021-01"))
         cls.define_version(Release("2021-04"))
         cls.define_version(Release("2021-07"))
+        cls.define_version(Release("2021-10"))
 
     @classmethod
     def clear_defined_versions(cls):