Test free-threaded Python with pytest-run-parallel

[cclauss]

Continues the work started in:

• #2627

To be confident about free-threaded compatibility, it is recommended to test with pytest-run-parallel.

• https://py-free-threading.github.io/testing

Something like: % uvx --with=pytest-run-parallel pytest --iterations=8 --parallel-threads=auto

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[peace-maker]

pwnlib isn't fully thread-safe so this might reveal some areas that need work.