Add support for free-threaded Python (PEP 703)

[Gaganraj2002]

Builds on #940 — this completes the remaining work for free-threaded Python support.

Changes

Rust (rust/src/python/mod.rs):

• Add gil_used = false to #[pymodule] declaration (PyO3 0.27 defaults to true)
• The Rust codebase has zero global mutable state, zero unsafe blocks, and zero static variables — all functions are pure computations, making it inherently thread-safe

Test workflow (.github/workflows/tests.yml):

• Add 3.13t and 3.14t to the test matrix
• Skip mypy/typing dependencies for free-threaded builds (not yet compatible)

Release workflow (.github/workflows/release.yml):

• Add a free-threaded wheel build step (--interpreter 3.13t 3.14t) in the build job
• Skipped for PyPy matrix entries

Tests:

• Add tests/test_thread_safety.py with concurrent stress tests for parse, now, duration, diff, and format (8 threads x 200 iterations each)

Test plan

• All 5 thread-safety tests pass locally
• Builds successfully with maturin on CPython 3.13
• CI: Tests pass on 3.13t and 3.14t
• CI: Free-threaded wheels build across all platforms

[Secrus]

Skip mypy/typing dependencies for free-threaded builds (not yet compatible)

Do you know of an issue tracking the support for that?

[codspeed-hq]

Merging this PR will not alter performance

✅ 1 untouched benchmark

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_{Comparing Gaganraj2002:free-threaded-support (43c7ba8) with master (cbab60c)}

[Gaganraj2002]

hey! the main tracking was mypyc/mypyc#1038 — it's closed now since basic support landed. mypy ships cp314t wheels but cp313t still doesn't have them. and even on 3.14t the mypyc support is still described as "not memory safe" under concurrent access by JukkaL himself (Sep 2025 comment on that issue). so figured safer to just skip it for now instead of dealing with potential flaky CI.

[Secrus]

Hey. Let's remove 3.13t from the builds and support. It has been deemed experimental by the CPython, and free-threading should be done only from 3.14

[Gaganraj2002]

Done.

[Secrus]

The new test file is missing type annotations. If you don't feel like adding them, you can add them to the list in pyproject.toml

[Gaganraj2002]

for now i have added it into the pyproject.toml
later when i get sometime will push another pr fixing the annotations.