Challenge 1: Add a Python Version Matrix — Possible Solution
====================================================================

jobs:
  test:
    strategy:
      matrix:
        python-version: ['3.10', '3.11', '3.12']
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-python@v5
        with:
          python-version: ${{ matrix.python-version }}
      - run: pip install -r requirements.txt
      - run: pytest

WHY THIS WORKS
--------------
- The strategy: matrix: block follows the exact same structure as this
  chapter's own Node.js example — just with python-version as the
  matrix key instead of node-version, and the three version numbers
  (3.10, 3.11, 3.12) listed as this challenge specifies.
- Each version is quoted as a STRING ('3.10' rather than 3.10) — this
  is a genuinely important YAML detail for version numbers like this:
  unquoted, 3.10 could be interpreted as the number 3.1 (YAML parsers
  can treat trailing zeros in a decimal as insignificant), silently
  dropping the distinction between 3.10 and 3.1. Quoting forces it to
  be treated as literal text, avoiding that specific parsing trap.
- ${{ matrix.python-version }} is used inside actions/setup-python's
  with: block, exactly mirroring how this chapter's example used
  ${{ matrix.node-version }} inside actions/setup-node — this is the
  templating syntax that lets GitHub Actions substitute in whichever
  specific version this particular parallel job run is handling.
- This produces exactly three parallel jobs — one for each Python
  version — just as the chapter's own three-Node-version example
  produced three parallel jobs, catching any "works on 3.12 but breaks
  on 3.10" compatibility bug before it reaches real users, rather than
  only testing whichever single version happens to be installed on a
  developer's own machine.