Support Python 3.14(t) and build 3.14(t) wheels

[ngoldbaum]

If you update to cibuildwheel 3.1 or newer, you should automatically generate Python 3.14 and 3.14t wheels.

It looks like you're using the deadsnakes action to get a free-threaded python build in the integration workflow. That's no longer necessary since setup-python supports the free-threaded build so you can switch to using setup-python and integrate the 3.13t and 3.14t CI and delete the special nogil CI job.

Let me know if you need a hand, I'm happy to help out.

c.f. #813