Use checkout instead of SDist

The Sdist is lacking tox.ini :(

.github/workflows/ci-supported-pythons.yml

@@ -60,19 +60,20 @@ jobs:
           name: Packages
           path: dist
 
-      - name: Prepare tests & config
+      # We have to check the project out because it doesn't include tox.ini in
+      # its SDist. If you include it, you can just unpack your SDist instead.
+      - uses: actions/checkout@v4
+        with:
+          repository: pypa/sampleproject
+
+      - name: Prepare for tox
         run: |
           # We use tox together with the fast tox-uv plugin.
           python -Im pip install tox-uv
 
-          # Unpack SDist for tests & config files.
-          tar xf dist/*.tar.gz --strip-components=1
-
           # Ensure tests run against wheel.
           rm -rf src
 
-      - run:
-          tree .
-          python -Im tox run --installpkg dist/*.whl -f py$(echo ${{ matrix.python-version }} | tr -d .)
+      - run: python -Im tox run --installpkg dist/*.whl -f py$(echo ${{ matrix.python-version }} | tr -d .)
 
 ...