ci: add free-threaded Python to CI

[henryiii]

Adding free-threaded Python. Adding a little to the test printout to make it easier to ensure the correct Python is being used.

[henryiii]

Actually, since we are testing now, let's add the trove classifier, too. Will add in a second.

[henryiii]

I added the classifier. I did do a quick stress test:

+@nox.session(python=["3.13t", "3.14t"])
+def testsft(session: nox.Session) -> None:
+    """
+    Run the tests multiple times in threads, stress test.
+    """
+    session.install(*nox.project.dependency_groups(PYPROJECT, "test"), "pytest-run-parallel")
+    session.install("-e.")
+
+    session.run(
+        "python",
+        "-m",
+        "pytest",
+        "--parallel-threads=2",
+        "--iterations=2",
+        *session.posargs,
+        env={"PYTEST_RUN_PARALLEL_VERBOSE": "1"},
+    )
+
+

That doesn't run quite a few tests due to monkeypatch not being threadsafe, and it fails on the musllinux tests, even though those have a monkey patch, so I guess it's running them in sequence (?), and due to caching, that causes an expected call to fail. So only test-related issues, packaging itself seems fine.

[ngoldbaum]

Thanks for handling this Henry!