Investigate PEP-703 python 3.13t free-threading

While much of the rest of the jupyter stack is blocked by difficult upstreams, traitlets likely could be test against 3.13t.

[pytest-run-parallel](https://github.com/Quansight-Labs/pytest-run-parallel) could be added semi-transparently to the existing test suite to start finding issues.

Start by reviewing the existing test suite and the pytest-run-parallel project, then run the tests under Python 3.13t to identify compatibility issues. Done means the feasibility of testing traitlets with free-threading is established, with any failures or required follow-up documented.

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