Use Python 3.13 as the default Python version for new apps (#1715)

Previously if a new app did not specify a Python version (e.g. via a
`.python-version` file), then the buildpack would default to the latest
Python 3.12 release.

Now the buildpack will use the latest Python 3.13 release.

This does not affect:
* apps that specify an explicit Python version
* existing apps that have already had a build using the previous default
  version (since that version is remembered from one build to the next,
  via the "version pinning" / "sticky versions" feature).

Use-cases involving new apps that aren't compatible with Python 3.13
will now need to specify Python 3.12 explicitly:
https://devcenter.heroku.com/articles/python-support#specify-a-python-version

However most apps should be compatible, since Python 3.13 was released
a couple of months ago (2024-10-07), does not contain significant breaking
changes over Python 3.12, and has been happily used by the
python-getting-started project since 2024-10-10.

See:
https://docs.python.org/3.13/whatsnew/3.13.html

GUS-W-14846972.

Author: edmorley

.github/workflows/build_python_runtime.yml

@@ -5,7 +5,7 @@ on:
   workflow_dispatch:
     inputs:
       python_version:
-        description: "Python version (eg: 3.12.0)"
+        description: "Python version (eg: 3.13.0)"
         type: string
         required: true
       stack:

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- Changed the default Python version for new apps from 3.12 to 3.13. ([#1715](https://github.com/heroku/heroku-buildpack-python/pull/1715))
 - Changed Python version pinning behaviour for apps that do not specify a Python version. Repeat builds are now pinned to the major Python version only (`3.X`) instead of the full Python version (`3.X.Y`), so that they always use the latest patch version. ([#1714](https://github.com/heroku/heroku-buildpack-python/pull/1714))
 
 ## [v269] - 2024-12-04

README.md

@@ -37,7 +37,7 @@ If none of those are found, the buildpack will use a default Python version for
 build of an app, and then subsequent builds of that app will be pinned to that version
 unless the build cache is cleared or you request a different version.
 
-The current default Python version is: 3.12
+The current default Python version is: 3.13
 
 The supported Python versions are:
 

lib/python_version.sh

@@ -11,7 +11,7 @@ LATEST_PYTHON_3_11="3.11.11"
 LATEST_PYTHON_3_12="3.12.8"
 LATEST_PYTHON_3_13="3.13.1"
 
-DEFAULT_PYTHON_FULL_VERSION="${LATEST_PYTHON_3_12}"
+DEFAULT_PYTHON_FULL_VERSION="${LATEST_PYTHON_3_13}"
 DEFAULT_PYTHON_MAJOR_VERSION="${DEFAULT_PYTHON_FULL_VERSION%.*}"
 
 # Integer with no redundant leading zeros.

spec/fixtures/ci_pipenv/.python-version

@@ -1 +1 @@
-3.12
+3.13

spec/fixtures/ci_pipenv/Pipfile.lock

@@ -1 +1 @@
-3.12
+3.13

spec/fixtures/ci_poetry/.python-version

@@ -2,7 +2,7 @@
 package-mode = false
 
 [tool.poetry.dependencies]
-python = "^3.12"
+python = "^3.13"
 typing-extensions = "*"
 
 [tool.poetry.group.test.dependencies]

spec/fixtures/ci_poetry/poetry.lock

@@ -1 +1 @@
-3.12
+3.13