IO from python-spawned threads causes deadlocks

**Description**

Carrying over from [#539](https://github.com/JuliaPy/PythonCall.jl/issues/539#issuecomment-2293753477)

The following code reaches a deadlock:

```python
from concurrent.futures import ThreadPoolExecutor, as_completed
from juliacall import Main as jl

jl.seval(
"""
function my_sqr()
println("Julia get's stuck at this print statement issued from thread $(Threads.threadid())")
a = rand(10)
for ii in 1:size(a, 1)
a[ii] = a[ii]^2
end
return sum(a)
end
"""
)

pool = ThreadPoolExecutor(2)

fs = {pool.submit(jl.my_sqr): ix for ix in range(10)}
for future in as_completed(fs):
print("running")
rank = fs[future]
results = future.result()
print("done")
print(results)
```

**Potential Fix**

Adding `ccall(:jl_enter_threaded_region, Cvoid, ())` as suggested in https://github.com/JuliaPy/PythonCall.jl/issues/539#issuecomment-2295200264 fixes this issue but not the original one of #539.
PythonCall may consider to invoke `jl_enter_threaded_region` behind the scenes.

Start by running the provided Python ThreadPoolExecutor example with juliacall and confirm where execution deadlocks. Then inspect the PythonCall path used for Python-spawned calls and the suggested jl_enter_threaded_region invocation; done means eliminating this deadlock without regressing the related behavior described in #539.

索引モデルが issue の本文から書いたものです。