Temporarily disable Python 3.9 for fastai example

Author: c-bata

.github/workflows/fastai.yml

@@ -14,7 +14,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ['3.9', '3.10', '3.11', '3.12']
+        # TODO(c-bata): Add Python 3.9 here after fixing https://github.com/optuna/optuna-examples/issues/307
+        python-version: ['3.10', '3.11', '3.12']
 
     steps:
     - uses: actions/checkout@v4