Add testing coverage for the free-threaded build

Author: ngoldbaum

.github/workflows/tests.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
+        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12", "3.13", "3.13t", "3.14", "3.14t"]
         os: [windows-latest, ubuntu-latest, macos-latest]
 
     env:
@@ -48,3 +48,9 @@ jobs:
         python -m pip install -U pip setuptools wheel
         python -m pip install -e .[test]
         python -m unittest -v tests.suite
+
+    - name: Test (free-threading stress test)
+      if: ${{ (steps.release.outputs.version == 0) && endsWith(matrix.python-version, 't') }}
+      run: |
+        python -m pip install unittest-ft
+        unittest-ft -s -v