Set up CI for the free threaded build

.github/workflows/ci.yml

@@ -161,6 +161,12 @@ jobs:
             python-version: pypy-3.10
             backend: c
             env: { NO_CYTHON_COMPILE: 1 }
+          # Free-threading
+          - os: ubuntu-20.04
+            python-version: 3.13-freethreading-dev
+            backend: "c,cpp"
+            env: {}
+            allowed_failure: true
 
     # This defaults to 360 minutes (6h) which is way too long and if a test gets stuck, it can block other pipelines.
     # From testing, the runs tend to take ~20 minutes for ubuntu / macos and ~40 for windows,
@@ -186,9 +192,17 @@ jobs:
 
       - name: Setup python
         uses: actions/[email protected]
+        if: "!endsWith(matrix.python-version, '-freethreading-dev')"
         with:
           python-version: ${{ matrix.python-version }}
 
+      - name: Setup python from deadsnakes
+        uses: deadsnakes/[email protected]
+        if: "endsWith(matrix.python-version, '-freethreading-dev')"
+        with:
+          python-version: 3.13-dev
+          nogil: true
+
       - name: Compilation Cache
         uses: hendrikmuhs/[email protected]
         with:

Tools/ci-run.sh

@@ -216,6 +216,9 @@ if [[ $TEST_CODE_STYLE != "1" ]]; then
 fi
 
 export CFLAGS="$CFLAGS $EXTRA_CFLAGS"
+if [[ $PYTHON_VERSION == *"-freethreading-dev" ]]; then
+  export PYTHON_GIL=0
+fi
 python runtests.py \
   -vv $STYLE_ARGS \
   -x Debugger \