ci: Add 3.15 in CI

[clin1234]

No description provided.

[davidhewitt]

I added full build, if you want to fixup merge conflict we can see what initial results are.

[bschoenmaeckers]

You should bump cpython’s max-version in pyo3-ffi/cargo.toml to 1.15.

[Tpt]

You should bump cpython’s max-version in pyo3-ffi/cargo.toml to 1.15.

cpython 3.15 C-API is I think still moving, wouldn't it better to keep the error message and use UNSAFE_PYO3_SKIP_VERSION_CHECK until the API is stabilized? I am a bit afraid we ship a PyO3 version that end up broken on cpython 3.15 released versions