Add CI job for free-threaded build

.github/workflows/ci.yml

@@ -489,6 +489,27 @@ jobs:
           echo PYO3_CONFIG_FILE=$PYO3_CONFIG_FILE >> $GITHUB_ENV
       - run: python3 -m nox -s test
 
+  test-free-threaded: 
+    if: ${{ contains(github.event.pull_request.labels.*.name, 'CI-build-full') || github.event_name != 'pull_request' }}
+    needs: [fmt]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: Swatinem/rust-cache@v2
+        with:
+          save-if: ${{ github.event_name != 'merge_group' }}
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rust-src
+    # TODO: replace with setup-python when there is support
+    - uses: deadsnakes/action@6c8b9b82fe0b4344f4b98f2775fcc395df45e494 # v3.1.0
+      with:
+          python-version: '3.13-dev'
+          nogil: true
+    - run: python3 -m sysconfig
+    - run: python3 -m pip install --upgrade pip && pip install nox
+    - run: python3 -m nox -s test
+
   test-version-limits:
     needs: [fmt]
     if: ${{ contains(github.event.pull_request.labels.*.name, 'CI-build-full') || github.event_name != 'pull_request' }}