ci: pin tests to Python 3.11 only

The installer hardcodes PYTHON_VERSION=3.11 and creates the venv
with that version. No point testing 3.12 — halves CI time.

.github/workflows/tests.yml

@@ -15,11 +15,6 @@ jobs:
   test:
     runs-on: ubuntu-latest
     timeout-minutes: 10
-    strategy:
-      matrix:
-        python-version: ['3.11', '3.12']
-      fail-fast: false
-
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -27,12 +22,12 @@ jobs:
       - name: Install uv
         uses: astral-sh/setup-uv@v5
 
-      - name: Set up Python ${{ matrix.python-version }}
+      - name: Set up Python 3.11
-        run: uv python install ${{ matrix.python-version }}
+        run: uv python install 3.11
 
       - name: Install dependencies
         run: |
-          uv venv .venv --python ${{ matrix.python-version }}
+          uv venv .venv --python 3.11
           source .venv/bin/activate
           uv pip install -e ".[all,dev]"