chore: bump version to 3.5.0

[Diego Barajas (barajas-d)]

Summary

• Bump version from `3.4.0` to `3.5.0`
• Update `pyproject.toml`, `mercadopago/config/config.py`, `CHANGELOG.md`

Changelog entry

Changed

• Item and shipment classes: renamed to generic classes without `Order` prefix (#145)
• Payer classes: consolidated into a single generic `payer.py` (#145)

CI

• Add Python 3.13 and 3.14 to CI matrix (#145)
• Drop Python 3.9 from CI matrix — not supported by this SDK (#145)
• Standardize CI/CD workflows (#145)

Test plan

• CI passes
• Version string is correct in all files