ci: derive Python versions from pyproject.toml requires-python

Replace hardcoded Python version matrix with a resolve-python job that
reads requires-python from pyproject.toml. Tests run on min (from
pyproject.toml) and max (runner's latest CPython). Lint runs on max only.

Author: schultzp2020

.github/workflows/ci.yml

@@ -10,7 +10,36 @@ permissions:
   contents: read
 
 jobs:
+  resolve-python:
+    runs-on: ubuntu-latest
+    outputs:
+      min-version: ${{ steps.resolve.outputs.min }}
+      max-version: ${{ steps.resolve.outputs.max }}
+      matrix: ${{ steps.resolve.outputs.matrix }}
+    steps:
+      - uses: actions/checkout@v4
+
+      - id: resolve
+        run: |
+          MIN=$(python3 -c "
+          import re
+          with open('pyproject.toml') as f:
+              m = re.search(r'requires-python\s*=\s*\">=(\d+\.\d+)\"', f.read())
+              print(m.group(1) if m else '3.9')
+          ")
+          # Latest stable CPython minor version
+          MAX=$(python3 -c "import sys; print(f'{sys.version_info.major}.{sys.version_info.minor}')")
+          if [ "$MIN" = "$MAX" ]; then
+            MATRIX="[\"$MIN\"]"
+          else
+            MATRIX="[\"$MIN\", \"$MAX\"]"
+          fi
+          echo "min=$MIN" >> "$GITHUB_OUTPUT"
+          echo "max=$MAX" >> "$GITHUB_OUTPUT"
+          echo "matrix=$MATRIX" >> "$GITHUB_OUTPUT"
+
   lint:
+    needs: resolve-python
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
@@ -21,7 +50,7 @@ jobs:
 
       - uses: actions/setup-python@v5
         with:
-          python-version: "3.11"
+          python-version: ${{ needs.resolve-python.outputs.max-version }}
 
       - run: uv sync --frozen --extra dev
 
@@ -30,11 +59,11 @@ jobs:
       - run: uv run ruff format --check .
 
   test:
-    needs: lint
+    needs: [resolve-python, lint]
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.9", "3.11"]
+        python-version: ${{ fromJson(needs.resolve-python.outputs.matrix) }}
     steps:
       - uses: actions/checkout@v4