Add free-threaded Python builds to CI matrix (#198)

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.github/workflows/main.yml

@@ -36,6 +36,7 @@ jobs:
         python:
         - "3.10"
         - "3.13"
+        - "3.13t"
         platform:
         - ubuntu-latest
         - macos-latest
@@ -47,6 +48,8 @@ jobs:
           platform: ubuntu-latest
         - python: "3.14"
           platform: ubuntu-latest
+        - python: "3.14t"
+          platform: ubuntu-latest
         - python: "3.15"
           platform: ubuntu-latest
         - python: pypy3.10