Add free-threaded Python 3.14 to CI

Not sure this _actually_ works with our dependencies, but worth a shot.

Author: stefanvanburen

.github/workflows/ci.yaml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14", "3.14t"]
         resolution: ["highest", "lowest-direct"]
     env:
       # Shared env variables for all the tests