Add CPython 3.15.0rc1

[jsirois]

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Summary by cubic

Add CPython 3.15.0rc1 to python-build so it can be installed with pyenv. Updates the 3.15 definition to use rc1 tarballs (.xz and .tgz) with new SHA256 checksums, replacing 3.15.0b4.

^{Written for commit 3f3bc60. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}