Add support for Python 3.13.8 (#1928)

Release announcement:
https://blog.python.org/2025/10/python-3138-is-now-available.html

Changelog:
https://docs.python.org/release/3.13.8/whatsnew/changelog.html#python-3-13-8

Binary builds:
https://github.com/heroku/heroku-buildpack-python/actions/runs/18379072906

GUS-W-17604362.

Author: edmorley

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## [Unreleased]
 
 - Added support for Python 3.14. ([#1927](https://github.com/heroku/heroku-buildpack-python/pull/1927))
+- The Python 3.13 version alias now resolves to Python 3.13.8. ([#1928](https://github.com/heroku/heroku-buildpack-python/pull/1928))
 - Stopped using `--with-system-expat` when compiling new Python versions. ([#1925](https://github.com/heroku/heroku-buildpack-python/pull/1925))
 
 ## [v312] - 2025-10-05

lib/python_version.sh

@@ -8,7 +8,7 @@ LATEST_PYTHON_3_9="3.9.23"
 LATEST_PYTHON_3_10="3.10.18"
 LATEST_PYTHON_3_11="3.11.13"
 LATEST_PYTHON_3_12="3.12.11"
-LATEST_PYTHON_3_13="3.13.7"
+LATEST_PYTHON_3_13="3.13.8"
 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 = '3.10.18'
 LATEST_PYTHON_3_11 = '3.11.13'
 LATEST_PYTHON_3_12 = '3.12.11'
-LATEST_PYTHON_3_13 = '3.13.7'
+LATEST_PYTHON_3_13 = '3.13.8'
 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+$/, '')