Migrate from runtime.txt to .python-version

[Copilot]

Heroku deprecated runtime.txt in favor of .python-version. Using major version only enables automatic patch updates.

Changes

• Removed runtime.txt (was pinned to python-3.12.6)
• Added .python-version with 3.12

App will now receive Python 3.12 patch updates automatically on each build.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Migrate runtime.txt to .python-version</issue_title>
<issue_description>We are currently using Python 3.12.6 as specified in runtime.txt. Heroku has deprecated the runtime.txt file and recommends switching to a .python-version file:

-----> Building on the Heroku-22 stack
-----> Using buildpacks:
       1. https://github.com/heroku/heroku-geo-buildpack.git
       2. heroku/python
-----> Geo Packages (GDAL/GEOS/PROJ) app detected
-----> GDAL_VERSION is not set. Using the same version as the last build: 3.5.0.
-----> GEOS_VERSION is not set. Using the same version as the last build: 3.10.2.
-----> PROJ_VERSION is not set. Using the same version as the last build: 8.2.1.
-----> Installing GDAL-3.5.0
-----> Installing GEOS-3.10.2
-----> Installing PROJ-8.2.1
-----> Python app detected
-----> Using Python 3.12.6 specified in runtime.txt
 !     Warning: The runtime.txt file is deprecated.
 !     
 !     The runtime.txt file is deprecated since it has been replaced
 !     by the more widely supported .python-version file:
 !     https://devcenter.heroku.com/changelog-items/3141
 !     
 !     Please switch to using a .python-version file instead.
 !     
 !     Delete your runtime.txt file and create a new file in the
 !     root directory of your app named:
 !     .python-version
 !     
 !     Make sure to include the '.' character at the start of the
 !     filename. Don't add a file extension such as '.txt'.
 !     
 !     In the new file, specify your app's major Python version number
 !     only. Don't include quotes or a 'python-' prefix.
 !     
 !     For example, to request the latest version of Python 3.12,
 !     update your .python-version file so it contains exactly:
 !     3.12
 !     
 !     We strongly recommend that you don't specify the Python patch
 !     version number, since it will pin your app to an exact Python
 !     version and so stop your app from receiving security updates
 !     each time it builds.
 !     
 !     In the future support for runtime.txt will be removed and
 !     this warning will be made an error.
 !     Warning: A Python patch update is available!
 !     
 !     Your app is using Python 3.12.6, however, there is a newer
 !     patch release of Python 3.12 available: 3.12.12
 !     
 !     It is important to always use the latest patch version of
 !     Python to keep your app secure.
 !     
 !     Update your runtime.txt file to use the new version.
 !     
 !     We strongly recommend that you don't pin your app to an
 !     exact Python version such as 3.12.6, and instead only specify
 !     the major Python version of 3.12 in your runtime.txt file.
 !     This will allow your app to receive the latest available Python
 !     patch version automatically and prevent this warning.
-----> Discarding cache since:
       - The contents of Pipfile.lock changed
-----> Installing Python 3.12.6
-----> Installing Pipenv 2026.0.3
-----> Installing dependencies using 'pipenv install --deploy'
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Migrate runtime.txt to .python-version #276

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