Add 3.13t CI using pytest-run-parallel

[ngoldbaum]

c.f. #1784.

This PR updates the tests to be runnable in a thread pool under pytest-run-parallel. It then makes use of pytest-run-parallel to do multithreaded testing on the free-threaded build in CI.

The updates to the tests are mostly to avoid sharing filesystem paths between test runners. I also marked all the multiprocessing tests as thread-unsafe because they rely on fork and they rely on mutating os.environ.

To avoid running into runtime borrow-checker errors, I wrapped the internal state of the PyEncoder pyclass in a Mutex. I don't think it's possible for any panics to happen while mutexes are locked that weren't already possible and in all cases I think it's correct to immediately propagate panics with unlock() rather than trying to handle poisoned mutexes.