Add support for free-threaded CPython

[andfoy]

See #1110

This PR enables support for free-threaded CPython 3.13, most of the changes are related to testing, which enable parallel testing using pytest-run-parallel. Additionally, ThreadSanitizer checks were performed without any issue

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[crusaderky]

Superseded by https://github.com/google/sentencepiece/issues/1110#issuecomment-3024357525

[crusaderky]

@taku910 this PR was not fit for purpose. Please revert.