Add support for Python 3.13.9 (#1934)

Release announcement:
https://blog.python.org/2025/10/python-3139-is-now-available.html

Changelog:
https://docs.python.org/release/3.13.9/whatsnew/changelog.html#python-3-13-9-final

Binary builds:
https://github.com/heroku/heroku-buildpack-python/actions/runs/18524770867

GUS-W-19897888.

Author: edmorley

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- The Python 3.13 version alias now resolves to Python 3.13.9. ([#1934](https://github.com/heroku/heroku-buildpack-python/pull/1934))
 - Improved error messages and metrics for unhandled buildpack errors. ([#1933](https://github.com/heroku/heroku-buildpack-python/pull/1933))
 
 ## [v313] - 2025-10-09

lib/python_version.sh

@@ -8,7 +8,7 @@ LATEST_PYTHON_3_9="3.9.24"
 LATEST_PYTHON_3_10="3.10.19"
 LATEST_PYTHON_3_11="3.11.14"
 LATEST_PYTHON_3_12="3.12.12"
-LATEST_PYTHON_3_13="3.13.8"
+LATEST_PYTHON_3_13="3.13.9"
 LATEST_PYTHON_3_14="3.14.0"
 
 OLDEST_SUPPORTED_PYTHON_3_MINOR_VERSION=9

spec/spec_helper.rb

@@ -14,7 +14,7 @@
 LATEST_PYTHON_3_10 = '3.10.19'
 LATEST_PYTHON_3_11 = '3.11.14'
 LATEST_PYTHON_3_12 = '3.12.12'
-LATEST_PYTHON_3_13 = '3.13.8'
+LATEST_PYTHON_3_13 = '3.13.9'
 LATEST_PYTHON_3_14 = '3.14.0'
 DEFAULT_PYTHON_FULL_VERSION = LATEST_PYTHON_3_13
 DEFAULT_PYTHON_MAJOR_VERSION = DEFAULT_PYTHON_FULL_VERSION.gsub(/\.\d+$/, '')