Update changelog for 1.5.3 release (#180)

Author: bcwu

CHANGELOG.md

@@ -4,7 +4,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## Unreleased
+## [1.5.3] - 2021-05-06
 
 ### Added
 - Support for generating md5 file upload checksums, even if Python's `hashlib`