changelog: Add the release number.

Author: jorisv

CHANGELOG.md

@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [Unreleased]
 
+## [3.1.2] - 2023-11-09
+
 ### Added
 - Support Python 3.12 ([#391](https://github.com/stack-of-tasks/eigenpy/pull/391))