chore: bump pyo3

[vinitkumar]

Summary

• upgrade the Rust extension to pyo3 0.28.2 and bump the Rust package version to 0.2.0
• fix several Rust binding correctness issues so XML output matches the Python implementation more closely
• refactor the Rust writer to reduce duplicate dispatch logic and avoid recursive intermediate string allocations
• refresh benchmark numbers and README performance claims to reflect the latest Rust implementation

What changed

• prevent double-escaping when invalid XML keys fall back to name="..." attributes
• use Python str() for float rendering so values like 1.0 preserve Python-compatible formatting
• propagate iterator errors instead of silently dropping exceptions from try_iter()
• validate custom_root and raise ValueError for invalid XML element names
• replace repeated conversion paths with a single buffered write flow (write_value, write_dict_contents, write_list_contents)
• add direct buffer helpers for text escaping, attribute escaping, and CDATA emission to cut allocation overhead
• add targeted tests for escaping and CDATA helpers
• update README and benchmark docs with the new Rust speedups and current measurements

Validation

• Rust code formatted and clippy/test follow-up fixes applied in this branch
• benchmark and documentation values updated to match the current Rust extension behavior

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.89%. Comparing base (a70b9ef) to head (0061df7).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #276   +/-   ##
=======================================
  Coverage   95.89%   95.89%           
=======================================
  Files           5        5           
  Lines         463      463           
=======================================
  Hits          444      444           
  Misses         19       19           

Flag	Coverage Δ
unittests	95.89% <ø> (ø)

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