feat: migrate from runtime.txt to .python-version

Replace legacy runtime.txt with .python-version for better
tooling support and developer experience.

Changes:
- Add .python-version file with Python 3.13.3
- Update .gitignore to track .python-version
- Modify GitHub Actions to use python-version-file parameter
- Remove hardcoded PYTHON_VERSION env variable from CI
- Add Python Version Management section to README
- Add Dockerfile comments referencing .python-version
- Delete runtime.txt

Benefits:
- Automatic version switching with pyenv/asdf/mise
- Single source of truth for Python version
- Simplified CI/CD configuration
- Better alignment with modern Python ecosystem

Closes #433

Author: nanotaboada

.github/workflows/python-app.yml

@@ -12,9 +12,6 @@ on:
   pull_request:
     branches: [ master ]
 
-env:
-  PYTHON_VERSION: 3.13.3
-
 jobs:
   lint:
     runs-on: ubuntu-latest
@@ -25,10 +22,10 @@ jobs:
       - name: Lint commit messages
         uses: wagoid/commitlint-github-action@v6.2.1
 
-      - name: Set up Python ${{ env.PYTHON_VERSION }}
+      - name: Set up Python
         uses: actions/setup-python@v6.0.0
         with:
-          python-version: ${{ env.PYTHON_VERSION }}
+          python-version-file: '.python-version'
           cache: 'pip'
 
       - name: Install lint dependencies
@@ -51,10 +48,10 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v5.0.0
 
-      - name: Set up Python ${{ env.PYTHON_VERSION }}
+      - name: Set up Python
         uses: actions/setup-python@v6.0.0
         with:
-          python-version: ${{ env.PYTHON_VERSION }}
+          python-version-file: '.python-version'
           cache: 'pip'
 
       - name: Install test dependencies

.gitignore

@@ -86,7 +86,7 @@ ipython_config.py
 # pyenv
 #   For a library or package, you might want to ignore these files since the code is
 #   intended to run in multiple environments; otherwise, check them in:
-# .python-version
+# Track .python-version for consistent Python version across environments
 
 # pipenv
 #   According to pypa/pipenv#598, it is recommended to include Pipfile.lock in version control.
@@ -158,4 +158,4 @@ cython_debug/
 #  be found at https://github.com/github/gitignore/blob/main/Global/JetBrains.gitignore
 #  and can be added to the global gitignore or merged into this file.  For a more nuclear
 #  option (not recommended) you can uncomment the following to ignore the entire idea folder.
-#.idea/
+#.idea/

.python-version

@@ -0,0 +1 @@
+3.13.3

Dockerfile

@@ -2,6 +2,7 @@
 # Stage 1: Builder
 # This stage builds the application and its dependencies.
 # ------------------------------------------------------------------------------
+# Python version should match .python-version file (currently 3.13.3)
 FROM python:3.13.3-slim-bookworm AS builder
 
 WORKDIR /app
@@ -19,6 +20,7 @@ RUN pip wheel --no-cache-dir --wheel-dir=/app/wheelhouse -r requirements.txt
 # Stage 2: Runtime
 # This stage creates the final, minimal image to run the application.
 # ------------------------------------------------------------------------------
+# Python version should match .python-version file (currently 3.13.3)
 FROM python:3.13.3-slim-bookworm AS runtime
 
 WORKDIR /app

README.md

@@ -19,6 +19,12 @@ Proof of Concept for a RESTful API made with [Python 3](https://www.python.org/)
 
 _Figure: Simplified, conceptual project structure and main application flow. Not all dependencies are shown._
 
+## Python Version Management
+
+This project uses `.python-version` to specify the required Python version. If you use [pyenv](https://github.com/pyenv/pyenv), [asdf](https://asdf-vm.com/), or [mise](https://mise.jdx.dev/), the correct Python version will be automatically activated when you enter the project directory.
+
+Alternatively, ensure you have Python 3.13.3 (or the version specified in `.python-version`) installed.
+
 ## Install
 
 ```console