Prepare release v318

[heroku-linguist]

Commits since the last release:

• Improve .python-version and runtime.txt error handling (Improve .python-version and runtime.txt error handling #1958)

For the full diff, see the compare view:
v317...main

[edmorley]

$ make publish
Checking environment...
Checking buildpack versions in sync...
Extracting changelog entry for this release...

- Improved the error message shown when a `.python-version` file uses an unsupported file encoding. ([#1958](https://github.com/heroku/heroku-buildpack-python/pull/1958))
- Improved the error message shown when a `.python-version` or `runtime.txt` file contain an invalid Python version. ([#1958](https://github.com/heroku/heroku-buildpack-python/pull/1958))
- Added support for commented lines in `runtime.txt`, for parity with `.python-version`. ([#1958](https://github.com/heroku/heroku-buildpack-python/pull/1958))

Deploy as v318 [y/n]? y

Creating GitHub release...
https://github.com/heroku/heroku-buildpack-python/releases/tag/v318

Publishing to the Heroku Buildpack Registry...
Two-factor code: ********************************************
Publishing buildpack heroku/python... done
Publishing buildpack was successful

 Version Released At              Status    
 ─────── ──────────────────────── ───────── 
 318     2025-11-12T17:36:00.077Z published