Migrate generated project from runtime.txt to .python-version

Heroku now supports the `.python-version` file as an alternative to the
Heroku-specific (and now deprecated) `runtime.txt` file, and recommends
apps switch to using the former since it's more widely supported by other
tooling.

The `.python-version` file is supported by pyenv, uv, the GitHub Actions
`setup-python` action and many others.

In addition, the `.python-version` file supports the version being specified
as just the major version (eg `3.12` vs `3.12.N`) which means will pick up
Python patch updates automatically (which is the recommended usage on
Heroku).

See:
https://devcenter.heroku.com/changelog-items/3005
https://github.com/heroku/heroku-buildpack-python/issues/1642
https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#using-the-python-version-file-input

hooks/post_gen_project.py

@@ -76,7 +76,7 @@ def remove_utility_files():
 
 
 def remove_heroku_files():
-    file_names = ["Procfile", "runtime.txt", "requirements.txt"]
+    file_names = ["Procfile", "requirements.txt"]
     for file_name in file_names:
         if file_name == "requirements.txt" and "{{ cookiecutter.ci_tool }}".lower() == "travis":
             # don't remove the file if we are using travisci but not using heroku

{{cookiecutter.project_slug}}/.github/dependabot.yml

@@ -98,7 +98,7 @@ updates:
   # Enable version updates for Python/Pip - Production
   - package-ecosystem: 'pip'
     # Look for a `requirements.txt` in the `root` directory
-    # also 'setup.cfg', 'runtime.txt' and 'requirements/*.txt'
+    # also 'setup.cfg', '.python-version' and 'requirements/*.txt'
     directory: '/'
     # Every weekday
     schedule:

{{cookiecutter.project_slug}}/.github/workflows/ci.yml

@@ -28,7 +28,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: '3.12'
+          python-version-file: '.python-version'
 
       {%- if cookiecutter.open_source_license != 'Not open source' %}
       # Consider using pre-commit.ci for open source project
@@ -88,9 +88,9 @@ jobs:
       {%- else %}
 
       - name: Set up Python
-        uses: actions/setup-python@v4
+        uses: actions/setup-python@v5
         with:
-          python-version: '3.12'
+          python-version-file: '.python-version'
           cache: pip
           cache-dependency-path: |
             requirements/base.txt

{{cookiecutter.project_slug}}/.gitignore

@@ -59,9 +59,6 @@ docs/_build/
 # PyBuilder
 target/
 
-# pyenv
-.python-version
-
 {% if cookiecutter.use_celery == 'y' -%}
 # celery beat schedule file
 celerybeat-schedule

{{cookiecutter.project_slug}}/.python-version

@@ -0,0 +1 @@
+3.12

{{cookiecutter.project_slug}}/runtime.txt

@@ -1 +0,0 @@
-python-3.12.7