Mention change to python version support in CHANGELOG

Author: Ben Krikler

CHANGELOG.md

@@ -5,6 +5,9 @@ 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]
+### Removed
+- Testing against Python <= 3.5, PR #124
+
 ### Fixed
 - Fix handling of empty data chunks in BinnedDataframe stage, PR #124 [@BenKrikler](https://github.com/benkrikler)