Add CHANGELOG entry for adding support for Python 3.12 (#810)

Author: hugovk

CHANGELOG.md

@@ -4,6 +4,10 @@ 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/).
 
+## Unreleased
+
+- Add support for Python 3.12. (#807)
+
 ## 0.18.0 (September 8th, 2023)
 
 - Add support for HTTPS proxies. (#745, #786)