chore(langchain): drop Python 3.9 to prep for v1 (#32704)

Python 3.9 EOL is October 2025, so we're going to drop it for the v1
alpha release.

.github/scripts/check_diff.py

@@ -132,6 +132,8 @@ def _get_configs_for_single_dir(job: str, dir_: str) -> List[Dict[str, str]]:
 
     elif dir_ == "libs/langchain" and job == "extended-tests":
         py_versions = ["3.9", "3.13"]
+    elif dir_ == "libs/langchain_v1":
+        py_versions = ["3.10", "3.13"]
 
     elif dir_ == ".":
         # unable to install with 3.13 because tokenizers doesn't support 3.13 yet