[Ctorum]

Description

Adds support for Python 3.15 and upgrades the Rust binding (rust-base32) from PyO3 0.27.2 → 0.29.3, which is required to build against Python 3.15.

Main changes:

• CI (publish.yml): adds 3.15 to the build/publish matrix for Windows, macOS (ARM64 and x86_64), and Linux (x86_64 and aarch64).
• pyproject.toml: new classifier Programming Language :: Python :: 3.15.
• CONTRIBUTING.md: "latest tested" bumped to Python 3.15.
• rust-base32/Cargo.toml: pyo3 bumped from 0.27.2 to 0.29.
• rust-base32/Cargo.lock: updates pyo3, pyo3-ffi, pyo3-build-config, pyo3-macros, and pyo3-macros-backend to 0.29.3. The transitive dependencies autocfg, indoc, memoffset, rustversion, and unindent are no longer needed and were dropped from the lockfile.

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Notes

• PyO3 0.27 → 0.29: the jump spans two minor versions, so it's worth checking the PyO3 changelog for API changes. This diff doesn't touch any Rust source code, so the current code is expected to compile without changes. Please confirm in CI.
• Slimmer lockfile: the removal of indoc, memoffset, unindent, autocfg, and rustversion comes from PyO3 0.29 itself no longer depending on them, not from manual cleanup.
• Python 3.15 was added to the matrix on all platforms; keep an eye on CI time/cost and on the availability of the 3.15 runtime on the runners.

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