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

[edmorley]

Hi! I'm the Heroku Python owner :-)

Heroku now supports the .python-version file as an alternative to the Heroku-specific (and now deprecated) runtime.txt file, and we recommend 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://devcenter.heroku.com/articles/python-runtimes#selecting-a-runtime
heroku/heroku-buildpack-python#1642
https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#using-the-python-version-file-input

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Checklist:

• I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates

[luzfcb]

LGTM.

@edmorley Since you're likely more familiar with the recent changes at Heroku, does the cookiecutter-django Heroku deployment tutorial require any updates to align with current Heroku best practices?

https://github.com/cookiecutter/cookiecutter-django/blob/master/docs/3-deployment/deployment-on-heroku.rst

[edmorley]

@luzfcb Thank you for the review!

Since you're likely more familiar with the recent changes at Heroku, does the cookiecutter-django Heroku deployment tutorial require any updates to align with current Heroku best practices?

Ah good question! Glancing through it, it seems mostly fine - though I spotted a couple of small tweaks that would be worth making - for which I've opened #5656.

[browniebroke]

Thanks!