Remove 3.9 support from CI and tests (#15401)

tests/runtests.py

@@ -50,7 +50,7 @@ def main() -> None:
     parser.add_argument(
         "--python-version",
         default=None,
-        choices=("3.9", "3.10", "3.11", "3.12", "3.13", "3.14"),
+        choices=("3.10", "3.11", "3.12", "3.13", "3.14"),
         # We're using the oldest fully supported version because it's the most likely to produce errors
         # due to unsupported syntax, feature, or bug in a tool.
         help="Target Python version for the test (defaults to oldest supported Python version).",