Add matrix testing to conformance tests

Maybe ought to just move these into the "unit test" workflow...

Author: stefanvanburen

.github/workflows/conformance.yaml

@@ -18,6 +18,7 @@ jobs:
       fail-fast: false
       matrix:
         python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
+        resolution: ["highest", "lowest-direct"]
     steps:
       - name: Checkout code
         uses: actions/checkout@v5
@@ -29,7 +30,7 @@ jobs:
           go-version: stable
           cache: false
       - uses: astral-sh/setup-uv@v6
-      - name: Install Python ${{ matrix.python-version }}
-        run: uv python install ${{ matrix.python-version }}
+        with:
+          python-version: ${{ matrix.python-version }}
       - name: Test conformance
-        run: make conformance
+        run: UV_RESOLUTION=${{ matrix.resolution }} make conformance