chore: make threads free-threaded-compatible

[P403n1x87]

Description

We make sure that our periodic threads are compatible with the free-threaded builds of CPython.

[cit-pr-commenter-54b7da]

Codeowners resolved as

ddtrace/internal/_threads.cpp                                           @DataDog/apm-core-python

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-05-15 21:01:04

Comparing candidate commit d982433 in PR branch chore/threads-free-threaded with baseline commit 1ab9c6f in branch main.

Found 0 performance improvements and 4 performance regressions! Performance is the same for 595 metrics, 10 unstable metrics.

scenario:iast_aspects-re_expand_aspect

• 🟥 execution_time [+253.313µs; +296.225µs] or [+7.278%; +8.511%]

scenario:iastaspects-stringio_aspect

• 🟥 execution_time [+581.722µs; +623.330µs] or [+14.927%; +15.995%]

scenario:iastaspectsospath-ospathbasename_aspect

• 🟥 execution_time [+97.392µs; +103.739µs] or [+22.284%; +23.737%]

scenario:span-start

• 🟥 execution_time [+1.397ms; +1.555ms] or [+8.940%; +9.949%]

[chatgpt-codex-connector]

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[datadog-datadog-prod-us1-2]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

_{This comment will be updated automatically if new data arrives.
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