Update PyO3 to 0.27.1 and Rust edition to 2021

This updates PyO3 from 0.23.5 to 0.27.1, bringing several improvements:
- Python 3.14 support (currently in beta)
- Enhanced thread-safety guarantees
- Improved Bound API performance
- Latest bug fixes and security patches

Also updates Rust edition from 2018 to 2021 for better language features.

Breaking Changes Analysis:
- ✅ No code changes required
- The codebase already uses modern Bound<PyModule> API (introduced in 0.21)
- Does not use any deprecated APIs (downcast, with_gil, GILOnceCell, etc.)
- Simple #[pyfunction] and #[pymodule] usage is fully compatible

Changes across PyO3 versions 0.23 → 0.27:
- 0.24: FromPyObject trait rework (not used in this codebase)
- 0.25: Removed deprecated IntoPy/ToPyObject traits (not used)
- 0.26: Thread-safety requirements for #[pyclass] (not used)
- 0.27: Additional API improvements (no impact on current code)

Risk Assessment: VERY LOW
- No API changes needed
- Already using compatible patterns
- Comprehensive CI testing will validate

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

Cargo.toml

@@ -1,15 +1,15 @@
 [package]
 name = "foobar"
 version = "0.1.0"
-edition = "2018"
+edition = "2021"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 [lib]
 name = "foobar"
 crate-type = ["cdylib"]
 
 [dependencies]
-pyo3 = { version = "0.23.5", features = ["extension-module"] }
+pyo3 = { version = "0.27.1", features = ["extension-module"] }
 
 [package.metadata.maturin]
 python-source = "python"