feat: Add python315.def and python315t.def

[gaborbernat]

Adds python315.def and python315t.def, the .def files src/lib.rs needs to generate import libraries for CPython 3.15 and free-threaded 3.15.

Generated the same way update.yml's update-pythonxy job normally does, run on my fork against #103's dll.yml change (gaborbernat#1/gaborbernat#2 there show the actual bot runs): gendef against real python315.dll/python315t.dll collected via actions/setup-python with 3.15-dev/3.15t-dev. Each file has 2000+ exported symbols, consistent with the 3.14 ones.

The follow-up src/lib.rs (3, 15) match arm (mirroring #94 for 3.14) is already written and passes cargo test, clippy and rustfmt on both ubuntu-latest and windows-latest on my fork (branch upstream-lib-315), but opening that as its own PR against main right now would show these .def files as part of its diff too, since they're not merged yet. I'll open it once this lands.

[gaborbernat]

@messense could you take a look at this when you get a chance, thanks!

[gaborbernat]

Closing along with #103 — see the explanation there: we're already on pyo3 0.29.3 with raw-dylib, so this crate isn't blocking us anymore.