Update free-threading CI

[hugovk]

We dropped 3.13t wheels in #9535 because the upcoming cibuildwheel will drop them because they were experimental.

Let's also drop testing 3.13t in our CI.

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Also, we defensively set PYTHON_GIL=0 to make sure nothing re-enables the GIL. That was when first adding support in 2024, and now the ecosystem has much better free-threading support.

@ngoldbaum suggested removing that, and like NumPy, add a test to fail if something does re-enable it. If something fails, that helps identify something that needs free-threading support, and we can report it (most likely to @ngoldbaum :)

https://github.com/numpy/numpy/blob/3bea241a0fbf5610f876dc54d4caf7380f75471e/numpy/conftest.py#L103

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We were skipping the pyqt6 install when PYTHON_GIL=0, because it doesn't support free-threading yet. Now we're removing that env var, let's only install if a wheel exists.