feat: run tests against 3.14 free-threaded python

I'm looking to start experimenting with threads for parallel kind generation. To make this worthwhile, we need to support free-threaded python. It looks like tests already pass here, so this should be a no-brainer for now. I've done some basic usage testing as well, and everything seems to work (even with free-threaded python...which makes sense seeing as we don't use threads anywhere at the moment).

Author: bhearsum

taskcluster/kinds/test/kind.yml

@@ -42,7 +42,7 @@ tasks:
         matrix:
             set-name: "unit-py{matrix[python]}"
             substitution-fields: [description, run.command, treeherder, worker]
-            python: ["314", "313", "312", "311", "310", "39"]
+            python: ["314t", "314", "313", "312", "311", "310", "39"]
         worker:
             artifacts:
                 - type: file