Track Python 3.15 CI support

[jonathan343]

Summary

I attempted to add Python 3.15 CI support in #46, but the 3.15 test leg failed due to upstream dependency compatibility.

What happened

On Python 3.15, uv sync --group test could not use a prebuilt wheel for pydantic-core==2.41.5 and fell back to a source build. pydantic-core==2.41.5 does not provide a compatible cp315 wheel in this environment and building from source on Python 3.15.0a5 fails with PyO3/abi3-related compile errors.

Example failure:

• https://github.com/jonathan343/spotify-sdk/actions/runs/21805342079/job/62907658000?pr=46

Decision (for now)

• Do not require Python 3.15 in PR CI yet.
• Keep tracking upstream readiness and re-enable once dependency support lands.

In pydantic/pydantic-core#1848 a maintainer mentioned:

we typically don't add support for new Python versions until the first beta. Also the first step will be to have PyO3 supporting 3.15.

I'll revisit in early May when the first beta for Python 3.15 is released (according to PEP-790)