Add CI/wheel builder for the free-threaded build

.github/workflows/ci.yml

@@ -78,15 +78,15 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        python-version: ['3.10', '3.11', '3.12', '3.13', 'pypy-3.10']
+        python-version: ['3.10', '3.11', '3.12', '3.13', '3.13t', 'pypy-3.10']
     steps:
       - uses: actions/checkout@v4
       - name: Get history and tags for SCM versioning to work
         run: |
           git fetch --prune --unshallow
           git fetch --depth=1 origin +refs/tags/*:refs/tags/*
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v5
+        uses: Quansight-Labs/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
       - name: Install dependencies

.github/workflows/release.yml

@@ -107,6 +107,7 @@ jobs:
         if: matrix.manylinux_version == 'manylinux2014'
         env:
           CIBW_BUILD: "cp311-* cp312-* cp313-* pp310-*"
+          CIBW_ENABLE: python-freethreading
           CIBW_ARCHS_MACOS: x86_64 universal2 arm64
           CIBW_ARCHS_LINUX: ${{ matrix.archs }}
           CIBW_ARCHS_WINDOWS: auto64
@@ -120,6 +121,7 @@ jobs:
         if: runner.os == 'Windows' && matrix.archs == 'ARM64'
         env:
           CIBW_BUILD: "cp310-* cp311-* cp312-* cp313-*"
+          CIBW_ENABLE: python-freethreading
           CIBW_ARCHS_WINDOWS: ${{ matrix.archs }}
           # It is not yet possible to run ARM64 tests, only cross-compile them.
           CIBW_TEST_SKIP: "*-win_arm64"

py/src/kiwisolver.cpp

@@ -162,6 +162,7 @@ kiwisolver_methods[] = {
 
 PyModuleDef_Slot kiwisolver_slots[] = {
     {Py_mod_exec, reinterpret_cast<void*>( kiwi_modexec ) },
+    {Py_mod_gil, Py_MOD_GIL_NOT_USED},
     {0, NULL}
 };