Update CHANGELOG.md

Author: LiamConnors

CHANGELOG.md

@@ -4,6 +4,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [UNRELEASED]
 
+### Updated
+- Removed Python 3.6 and Python 3.7 support [[#4490](https://github.com/plotly/plotly.py/pull/4490)]
+
 ### Fixed
 - Ensure scatter `mode` is deterministic from `px` [[#4429](https://github.com/plotly/plotly.py/pull/4429)]
 - Fix issue with creating dendrogram in subplots [[#4411](https://github.com/plotly/plotly.py/pull/4411)],