Unbounded thread growth during pytest

[sarahec]

While building on Darwin (MacOS) for nixpkgs, the threads grow without limit in tests/ndarray/test_lazyexpr.py reaching over 11K threads (!).

The same code on Linux remains under 400 threads can spawn 3-5K threads.

Release 3.12.2, pytest 8.4.2

[sarahec]

You may need to pass max-workers to the threadpool executor.