update CHANGELOG.md

Author: KoyanagiHitoshi

CHANGELOG.md

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 # Change Log
 
+## [v3.12.0](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/releases/tag/v3.12.0) / 2023-12-03
+
+* [#33](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/pull/33) add code
+* [#33](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/pull/33) update CHANGELOG.md
+
 ## [v3.11.0](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/releases/tag/v3.11.0) / 2023-11-05
 
 * [#32](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/pull/32) add code