update CHANGELOG.md

Author: KoyanagiHitoshi

CHANGELOG.md

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 # Change Log
 
+## [v3.9.0](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/releases/tag/v3.9.0) / 2023-10-01
+
+* [#30](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/pull/30) add code
+* [#30](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/pull/30) update CHANGELOG.md
+
 ## [v3.8.0](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/releases/tag/v3.8.0) / 2023-09-24
 
 * [#29](https://github.com/KoyanagiHitoshi/AtCoder-Python-Introduction/pull/29) add code