update CHANGELOG.md

Author: KoyanagiHitoshi

CHANGELOG.md

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 # Change Log
 
+## [v3.31.0](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/releases/tag/v3.31.0) / 2025-02-23
+
+* [#52](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/pull/52) add code
+* [#52](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/pull/52) update CHANGELOG.md
+
 ## [v3.30.0](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/releases/tag/v3.30.0) / 2025-01-26
 
 * [#51](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/pull/51) add code