Update CHANGELOG.md

Author: cpsievert

CHANGELOG.md

@@ -5,7 +5,7 @@ All notable changes to Shiny for Python 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).
 
-## [1.5.0] - 2025-09-08
+## [UNRELEASED]
 
 ### New features