concurrency fails w/o warmup

trying to recapitulate the [multithreading example](https://juliapy.github.io/PythonCall.jl/stable/juliacall/#py-multi-threading) in the docs, and on ubuntu and macos i have to call `systemsleep` normally beforehand or submitting it to the pool will hang:

```
Python 3.14.0 | packaged by conda-forge | (main, Oct 12 2025, 19:48:32) [Clang 20.1.8 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from concurrent.futures import ThreadPoolExecutor, wait
>>> from juliacall import Main as jl
>>> pool = ThreadPoolExecutor(4)
>>> jl.Libc.systemsleep(1) # w/o this line the next line hangs
0
>>> f = pool.submit(jl.Libc.systemsleep, 5)
>>> f

>>> import juliacall
>>> juliacall.__version__
'0.9.28'
>>> jl.VERSION
Julia: v"1.12.0"
```

Run the documented PythonCall multithreading example on Ubuntu or macOS with Python 3.14, Julia 1.12, and PythonCall 0.9.28, first without the `systemsleep` warmup. Compare the behavior with and without that call; done means submitting `jl.Libc.systemsleep` to a `ThreadPoolExecutor` no longer hangs without warmup.

索引モデルが issue の本文から書いたものです。