release: Update CHANGELOG.md for 3.9.1

Author: ManifoldFR

CHANGELOG.md

@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [Unreleased]
 
+## [3.9.1] - 2024-09-19
+
 ### Added
 
 - Add test returning reference of std::pair ([#503](https://github.com/stack-of-tasks/eigenpy/pull/503))
@@ -683,7 +685,8 @@ One can now easily expose C++ struct containing Eigen objects in Python avoiding
 
 ## [1.0.0] - 2014-07-18
 
-[Unreleased]: https://github.com/stack-of-tasks/eigenpy/compare/v3.9.0...HEAD
+[Unreleased]: https://github.com/stack-of-tasks/eigenpy/compare/v3.9.1...HEAD
+[3.9.1]: https://github.com/stack-of-tasks/eigenpy/compare/v3.9.0...v3.9.1
 [3.9.0]: https://github.com/stack-of-tasks/eigenpy/compare/v3.8.2...v3.9.0
 [3.8.2]: https://github.com/stack-of-tasks/eigenpy/compare/v3.8.1...v3.8.2
 [3.8.1]: https://github.com/stack-of-tasks/eigenpy/compare/v3.8.0...v3.8.1