Support Python 3.14 (and drop Python 3.9)

[eriknw]

Also add support for free-threading Python (as beta)

[alugowski]

For Python 3.13 and 3.14 we need the newer cibuildwheel.

I briefly looked at the errors and saw two:

• The two failing linux jobs appeared to be because the SuiteSparse lib went to a lib64 directory instead of lib, and the repair tool couldn't find it. The fix is likely either specifying the lib dir explicitly or somehow telling auditwheel to check in more places. We'd have to dig in on that.
• The Windows 3.14t build appears to link against the wrong Python lib, python314.lib where I'm guessing it should link against python314t.lib. That sounds like not our bug and we'd have to wait for an upstream fix. We can simply skip py314t on windows.