Drop Python 3.9 from CI matrix and setup.py (EOL, breaks float|None syntax)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JihaoXin

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.9", "3.10", "3.11", "3.12"]
+        python-version: ["3.10", "3.11", "3.12"]
 
     steps:
       - uses: actions/checkout@v4

setup.py

@@ -26,5 +26,5 @@
             "ark=ark.cli:main",
         ],
     },
-    python_requires=">=3.9",
+    python_requires=">=3.10",
 )